chore(executor): print exception stacktrace in executor

[harshachinta]

In some scenarios during systest execution the error description is getting masked and is shown as null. This makes debugging the error harder. Adding an additional log to print the exception along with stack trace.
b/350177934

Example:
The below logging happens at this place https://github.com/googleapis/java-spanner/blob/main/google-cloud-spanner-executor/src/main/java/com/google/cloud/executor/spanner/CloudClientExecutor.java#L2669-L2673.

 2024-06-29 06:10:52.179275-07007   Jun 29, 2024 6:10:52 AM com.google.cloud.executor.spanner.CloudClientExecutor executeQuery
 2024-06-29 06:10:52.179291-07007   INFO: Executing query Writes2-auto-1719665201034008-429/4011
 2024-06-29 06:10:52.179298-07007   sql: "@{ spanner.require_enhance_query = true }\nSELECT\n  orderbyscan_67.a_4 AS col59,\n  orderbyscan_67.a_1 AS col57,\n  orderbyscan_67.a_62 AS col116,\n  orderbyscan_67.a_48 AS col102,\n  CAST(NULL AS BOOL) AS col119,\n  orderbyscan_67.a_63 AS col120\nFROM\n  (\n    SELECT\n      filterscan_66.a_1 AS a_1,\n      filterscan_66.a_4 AS a_4,\n      filterscan_66.a_48 AS a_48,\n      filterscan_66.a_62 AS a_62,\n      filterscan_66.a_63 AS a_63\n    FROM\n      (\n        SELECT\n          projectscan_5.a_1 AS a_1,\n          projectscan_5.a_4 AS a_4,\n          projectscan_64.a_48 AS a_48,\n          projectscan_64.a_62 AS a_62,\n          projectscan_64.a_63 AS a_63\n        FROM\n          (\n            SELECT\n              orderbyscan_3.a_1 AS a_1,\n              CAST(@p0 AS INT64) AS a_4\n            FROM\n              (\n                SELECT\n                  filterscan_2.a_1 AS a_1\n                FROM\n                  (\n                    SELECT\n                      SqlSearchSingers.SingerId AS a_1\n                    FROM\n                      SqlSearchSingers\n                    WHERE\n                      (SqlSearchSingers.SingerId) = 78\n                  ) AS filterscan_2\n                ORDER BY filterscan_2.a_1 DESC, filterscan_2.a_1 DESC\n              ) AS orderbyscan_3\n          ) AS projectscan_5\n          INNER JOIN\n          @{ spanner.join_method = \"loop_join\" }\n          (\n            SELECT\n              SqlSearchAlbums.Rating AS a_48,\n              SCORE_NGRAMS(SqlSearchAlbums.Tracks_Substring_Tokens, \"kOn4TcygPawzuArqyvONrxohAGBKoFupxii4Z\\nmDHnJwPS\") AS a_62,\n              DEBUG_TOKENLIST(SqlSearchAlbums.Rating_Tokens) AS a_63\n            FROM\n              SqlSearchAlbums @{ force_index = \"SqlSearchAlbumsNoDivSearchIndex\" }\n            WHERE\n              (((((((SqlSearchAlbums.AlternativeName) IN (b\"text full full full \", b\"FLqJgAwP\", b\"GdgBY2dlyiTHIxYH5C7vl3vKZqW4y\")) AND\n              ((SEARCH_SUBSTRING(SqlSearchAlbums.Tracks_Substring_Tokens, \"awzuArqyvONrxohAGBKoFu\")) OR (SEARCH_NGRAMS(SqlSearchAlbums.Tracks_Substring_Tokens,\n                \"kOn4TcygPawzuArqyvONrxohAGBKoFupxii4Z\\nmDHnJwPS\")))) AND ((SqlSearchAlbums.Rating) >= CAST(NULL AS FLOAT64))) AND\n              (SEARCH(SqlSearchAlbums.Text_Tokens, \'\"Spanner AROUND(8) Spanner search\"\', enhance_query  => true))) AND\n              (CAST(NULL AS INT64) <= (SqlSearchAlbums.Length))) AND (7537955017849946775 > (SqlSearchAlbums.Profit))) AND\n              ((SqlSearchAlbums.SingerId) = 78)\n          ) AS projectscan_64\n          ON (\n              SELECT\n                SAFE_CAST(JSON_VALUE(\'{\"array\":[null,1.3589607628108293e+308],\"bool\":false,\"null\":null,\"number_float\":1.7010656682977979e+308,\"number_integer\":2989478548941050409,\"number_unsigned\":14203771396237232853,\"object\":{\"array\":[-261631567938255465],\"string\":\"TiUFnQoWz9K5l8\"},\"string\":\"DeRGvNTSbmkwYf35mbCTq8mI54WfOiK0eu2whtJ:0hKd5IauIK:C3PlX6RTdRDg\"}\',\n                  \"$.bool\") AS BOOL) AS a_65\n              LIMIT 1 OFFSET 0)\n        WHERE\n          (projectscan_64.a_63) IN (\"tWkBDt1KlB:MWpk6Y9KQ67EHz:ULdJtcf2w\", CAST(NULL AS STRING))\n      ) AS filterscan_66\n    ORDER BY filterscan_66.a_4\n  ) AS orderbyscan_67;"
 2024-06-29 06:10:52.179306-07007   params {
 2024-06-29 06:10:52.179307-07007     name: "p0"
 2024-06-29 06:10:52.179312-07007     type {
 2024-06-29 06:10:52.179313-07007       code: STRING
 2024-06-29 06:10:52.179314-07007     }
 2024-06-29 06:10:52.179316-07007     value {
 2024-06-29 06:10:52.179317-07007       string_value: "K7tCW382wr0aysd2FhkYpMWl"
 2024-06-29 06:10:52.179318-07007     }
 2024-06-29 06:10:52.179319-07007   }
 2024-06-29 06:10:52.179320-07007   
 2024-06-29 06:10:52.179321-07007   
 2024-06-29 06:10:52.179365-07007   Jun 29, 2024 6:10:52 AM com.google.cloud.executor.spanner.CloudClientExecutor executeQuery
 2024-06-29 06:10:52.179367-07007   INFO: Finish query building, ready to execute Writes2-auto-1719665201034008-429/4011
 2024-06-29 06:10:52.179416-07007   
 2024-06-29 06:10:52.180071-07007   Jun 29, 2024 6:10:52 AM com.google.cloud.executor.spanner.CloudClientExecutor executeQuery
 2024-06-29 06:10:52.180083-07007   INFO: Parsing query result Writes2-auto-1719665201034008-429/4011
 2024-06-29 06:10:52.180086-07007   
 2024-06-29 06:10:52.180452-07007   Jun 29, 2024 6:10:52 AM com.google.cloud.executor.spanner.CloudClientExecutor processResults
 2024-06-29 06:10:52.180460-07007   INFO: Iterating result set: Writes2-auto-1719665201034008-429/4011
 2024-06-29 06:10:52.180463-07007   
 2024-06-29 06:10:52.235759-07007   Jun 29, 2024 6:10:52 AM com.google.cloud.executor.spanner.CloudClientExecutor processResults
 2024-06-29 06:10:52.235785-07007   WARNING: Encountered exception: : null Writes2-auto-1719665201034008-429/4011
 2024-06-29 06:10:52.235789-07007   
 2024-06-29 06:10:52.236111-07007   Jun 29, 2024 6:10:52 AM com.google.cloud.executor.spanner.CloudExecutor$OutcomeSender sendOutcome
 2024-06-29 06:10:52.236123-07007   INFO: Sending result status {
 2024-06-29 06:10:52.236126-07007     code: 13
 2024-06-29 06:10:52.236127-07007     message: ": null"