fix: allow SQLERRM in procedures when package has EXCEPTION variables

When a package declares user-defined EXCEPTION variables at package
scope, passing SQLERRM to procedures from exception handlers would fail
with "exception variables cannot be used in this context".

The root cause was in is_const_datum() which threw an error when
encountering EXCEPTION datums. This function is called when iterating
over datums during subprocedure calls to determine which variables need
to be copied between stack frames.

EXCEPTION variables are constant identifiers that don't need copying,
so returning true (constant) is the correct behavior - it causes callers
to skip them appropriately.

Fixes IvorySQL#1151

Author: rophy

src/pl/plisql/src/pl_package.c

@@ -3068,11 +3068,12 @@ is_const_datum(PLiSQL_execstate *estate, PLiSQL_datum *datum)
 			}
 			break;
 		case PLISQL_DTYPE_EXCEPTION:
-
-			/* Exception variables cannot be used here */
-
-			elog(ERROR, "exception variables cannot be used in this context");
-
+			/*
+			 * Exception variables are constant identifiers - they cannot be
+			 * assigned to. Return true to indicate they are constant and
+			 * should be skipped by callers that iterate over datums.
+			 */
+			isconst = true;
 			break;