Java CodeGen should provide clear message for self-service trouble shooting

[lirenhe]

Clear and concise description of the problem

We need to empower our customers both internal and external to easily trouble shooting and update/fix spec issues that would block the CodeGen.
If certain TypeSpec features or patterns are not supported by CodeGen, we should clearly point it out and suggest them to update the spec with the features we supported for CodeGen.

Today, Java CodeGen could already do more then just let emitter crash.
Create this issue for any further improvement needed for self-service trouble shooting.

Some proposals:

1. Leverage TypeSpec diagnostics APIs to report the errors and provide more context so that users could easily identify the TypeSpec source that cause the error..
2. Provide a clear message for the unsupported scenarios and ideally provide some suggestions on how to fix.

Checklist

• Follow our Code of Conduct
• Read the docs.
• Check that there isn't already an issue that request the same feature to avoid creating a duplicate.