Dispatch WebformGoogleSheetsErrorEvent with the actual exception message

Problem/Motivation

The logger receives the actual exception message, but WebformGoogleSheetsErrorEvent receives only a generic message. Event subscribers cannot inspect the real failure reason.

Proposed resolution

Pass $e->getMessage() or the captured $error into WebformGoogleSheetsErrorEvent.

Remaining tasks

• Error event contains the same meaningful error message as the log.
• No event name changes.
• Existing getErrorMessage() remains compatible.
• Add/adjust test coverage.