Improve error message when model base path is missing

[Sraman01-code]

This change improves the error message emitted when TensorFlow Serving
cannot find the configured model base path.

The new message explains the expected directory structure and helps users
diagnose common deployment and mounting issues more quickly.

Testing:

• Ran TensorFlow Serving with a valid model mount and verified normal operation.
• Removed the model base path at runtime to confirm the filesystem error path
  is exercised and the improved error message would be surfaced.