Reword SPI_ERROR_TRANSACTION errors in PL/pgSQL

The previous message for SPI_ERROR_TRANSACTION claimed "cannot begin/end
transactions in PL/pgSQL", but that is no longer true.  Nevertheless,
the error can still happen, so reword the messages.  The error cases in
exec_prepare_plan() could never happen, so remove them.

src/pl/plpgsql/src/expected/plpgsql_transaction.out

@@ -409,7 +409,7 @@ $$;
 INFO:  read committed
 INFO:  repeatable read
 INFO:  read committed
--- error case
+-- error cases
 DO LANGUAGE plpgsql $$
 BEGIN
     SET TRANSACTION ISOLATION LEVEL REPEATABLE READ;
@@ -418,6 +418,20 @@ $$;
 ERROR:  SET TRANSACTION ISOLATION LEVEL must be called before any query
 CONTEXT:  SQL statement "SET TRANSACTION ISOLATION LEVEL REPEATABLE READ"
 PL/pgSQL function inline_code_block line 3 at SET
+DO LANGUAGE plpgsql $$
+BEGIN
+    SAVEPOINT foo;
+END;
+$$;
+ERROR:  unsupported transaction command in PL/pgSQL
+CONTEXT:  PL/pgSQL function inline_code_block line 3 at SQL statement
+DO LANGUAGE plpgsql $$
+BEGIN
+    EXECUTE 'COMMIT';
+END;
+$$;
+ERROR:  EXECUTE of transaction commands is not implemented
+CONTEXT:  PL/pgSQL function inline_code_block line 3 at EXECUTE
 DROP TABLE test1;
 DROP TABLE test2;
 DROP TABLE test3;

src/pl/plpgsql/src/pl_exec.c

@@ -3965,27 +3965,8 @@ exec_prepare_plan(PLpgSQL_execstate *estate,
 							  (void *) expr,
 							  cursorOptions);
 	if (plan == NULL)
-	{
-		/* Some SPI errors deserve specific error messages */
-		switch (SPI_result)
-		{
-			case SPI_ERROR_COPY:
-				ereport(ERROR,
-						(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
-						 errmsg("cannot COPY to/from client in PL/pgSQL")));
-				break;
-			case SPI_ERROR_TRANSACTION:
-				ereport(ERROR,
-						(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
-						 errmsg("cannot begin/end transactions in PL/pgSQL"),
-						 errhint("Use a BEGIN block with an EXCEPTION clause instead.")));
-				break;
-			default:
 		elog(ERROR, "SPI_prepare_params failed for \"%s\": %s",
 			 expr->query, SPI_result_code_string(SPI_result));
-				break;
-		}
-	}
 	if (keepplan)
 		SPI_keepplan(plan);
 	expr->plan = plan;
@@ -4129,8 +4110,7 @@ exec_stmt_execsql(PLpgSQL_execstate *estate,
 		case SPI_ERROR_TRANSACTION:
 			ereport(ERROR,
 					(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
-					 errmsg("cannot begin/end transactions in PL/pgSQL"),
-					 errhint("Use a BEGIN block with an EXCEPTION clause instead.")));
+					 errmsg("unsupported transaction command in PL/pgSQL")));
 			break;
 
 		default:
@@ -4317,8 +4297,7 @@ exec_stmt_dynexecute(PLpgSQL_execstate *estate,
 		case SPI_ERROR_TRANSACTION:
 			ereport(ERROR,
 					(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
-					 errmsg("cannot begin/end transactions in PL/pgSQL"),
-					 errhint("Use a BEGIN block with an EXCEPTION clause instead.")));
+					 errmsg("EXECUTE of transaction commands is not implemented")));
 			break;
 
 		default:

src/pl/plpgsql/src/sql/plpgsql_transaction.sql

@@ -335,13 +335,25 @@ BEGIN
 END;
 $$;
 
--- error case
+-- error cases
 DO LANGUAGE plpgsql $$
 BEGIN
     SET TRANSACTION ISOLATION LEVEL REPEATABLE READ;
 END;
 $$;
 
+DO LANGUAGE plpgsql $$
+BEGIN
+    SAVEPOINT foo;
+END;
+$$;
+
+DO LANGUAGE plpgsql $$
+BEGIN
+    EXECUTE 'COMMIT';
+END;
+$$;
+
 DROP TABLE test1;
 DROP TABLE test2;
 DROP TABLE test3;