Inform user of unsupported scram auth method

SCRAM-SHA-256 was introduced[1] as an authentication method into
PostgreSQL 10. This commit only informs the user that this driver does
not (yet) support this authentication method.

[1] https://git.postgresql.org/gitweb/?p=postgresql.git;a=commitdiff;h=818fd4a67d610991757b610755e3065fb99d80a5

conn.go

@@ -1132,6 +1132,8 @@ func (cn *conn) auth(r *readBuf, o values) {
 		if r.int32() != 0 {
 			errorf("unexpected authentication response: %q", t)
 		}
+    case 10:
+        errorf("Server requests scram authentication method. The pq driver does not support this.")
 	default:
 		errorf("unknown authentication response: %d", code)
 	}