Fix/error code hints 215

[XXOriginalXX]

Summary

Fixes #215

When a script exits with a non-zero return code, MLCFlow now displays a
human-readable hint alongside the exit code to help users diagnose failures
faster — without needing to look up exit codes manually.

Changes

• Added _SCRIPT_EXIT_CODE_HINTS dict mapping common exit codes to advice
• Added _get_exit_code_hint() helper function (handles POSIX codes + curl network codes 6/7)
• Updated ScriptExecutionError to accept and store return_code, appending the hint to the exception message
• Updated call_script_module_function to pass result["return"] as return_code when raising
• Updated README with new Error Code Hints section and table
• Added GitHub Actions CI test

Testing

Tested locally on Windows. CI workflow included.
All existing tests pass.

[github-actions]

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[arjunsuresh]

Looks good. Thank you for the contribution!
Can you please remove any formatting for the existing code? - there's an autoformatter running in github action which will otherwise reformat the code again.

[XXOriginalXX]

Done applied autopep8 formatting to script_action.py.