vdk-dag: improve error handling and error messages

To make it easier to debug VDK DAG jobs some improvements in error
handling are done.
- Stop using deprecated UserCodeError in favour of domain oriented
exceptoin class
- Improve the formatting of the error message in case of failure.

Testing Done: existing tests for regression.
Here is a comparison before and now in the formatting:
https://gist.githubusercontent.com/antoniivanov/8f87cdef1c610fcd24361197bcba45dc/raw/c31b990de2c0b1f49ac5989419cfc6ab12aac934/gistfile1.txt

projects/vdk-plugins/vdk-dag/src/vdk/plugin/dag/exception.py

@@ -1,10 +1,5 @@
 # Copyright 2023-2024 Broadcom
 # SPDX-License-Identifier: Apache-2.0
-
-# Copyright 2021-2024 VMware, Inc.
-# SPDX-License-Identifier: Apache-2.0
-import json
-from typing import Any
 from typing import List
 from typing import Optional
 

projects/vdk-plugins/vdk-dag/tests/test_local_executor.py

@@ -23,7 +23,7 @@ def reduce_retries_in_test_http_requests():
     with mock.patch.dict(
         os.environ,
         {
-            "DAG_LOCAL_RUN_JOB_PATH": jobs_path_from_caller_directory(""),
+            "DAGS_LOCAL_RUN_JOB_PATH": jobs_path_from_caller_directory(""),
             "DAGS_DAG_EXECUTION_CHECK_TIME_PERIOD_SECONDS": "0",
             "DAGS_TIME_BETWEEN_STATUS_CHECK_SECONDS": "0",
         },