refactor(internal): improve error handling for user-facing messages

[julieqiu]

Error messages are updated throughout the internal/ directory (excluding internal/legacylibrarian and internal/sidekick) to be more user-friendly and consistent.

Changes include:

• Add quotes around variables values with %q
• Simplify wording and remove technical jargon
• Adopt "no X found" pattern for missing resources
• Change "must not be empty" to "must provide X" for validation errors
• Replace "not implemented" with "does not support" for unsupported operations

For https://github.com/googleapis/librarian/issues/3680c

[gemini-code-assist]

Summary of Changes

Hello @julieqiu, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request refactors error handling within the internal/ directory to significantly improve the user experience by making error messages more consistent, clear, and actionable. The changes focus on standardizing the language, adding context through quoting, and adopting specific patterns for common error scenarios like missing resources or validation failures. This effort aims to reduce confusion and provide better guidance to users encountering issues with the librarian tool.

Highlights

• Standardized Error Messaging: Error messages across the internal/ directory have been updated to be more user-friendly and consistent, adhering to a set of predefined patterns.
• Improved Error Clarity with Quoting: Variable values within error messages now consistently use %q for quoting, enhancing readability and clarity for user-facing messages.
• Simplified Wording: Technical jargon has been removed and wording simplified in many error messages to make them more accessible to end-users.
• Consistent 'No X Found' Pattern: Errors related to missing resources now follow a 'no X found' pattern, providing clearer feedback when an item is not located.
• Validation Error Messaging: Validation errors have been updated from 'must not be empty' to 'must provide X', offering more direct guidance on required inputs.
• Unsupported Operation Messaging: Messages for unsupported operations have been changed from 'not implemented' to 'does not support', providing a more accurate description of the limitation.
• Logging Library Change: The cmd/librarian/main.go file has switched from using log/slog for fatal errors to the standard log package, simplifying the error handling at the application entry point.

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[gemini-code-assist]

Code Review

This pull request does an excellent job of refactoring error messages to be more user-friendly and consistent, aligning with the goals outlined in the description. The changes are systematic and improve code quality by making errors more informative and, in some cases, programmatically inspectable.

I found one critical issue that will prevent the code from compiling and one medium-severity suggestion to further improve consistency in error handling. Once the critical issue is addressed, this will be a great contribution.

[codecov]

Codecov Report

❌ Patch coverage is 17.77778% with 37 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.44%. Comparing base (3dbe74f) to head (0ab9826).

Files with missing lines	Patch %	Lines
internal/librarian/python/generate.go	0.00%	9 Missing ⚠️
internal/librarian/generate.go	12.50%	7 Missing ⚠️
internal/librarian/rust/generate.go	0.00%	7 Missing ⚠️
internal/serviceconfig/serviceconfig.go	0.00%	5 Missing ⚠️
internal/librarian/bump.go	57.14%	1 Missing and 2 partials ⚠️
cmd/librarian/main.go	0.00%	1 Missing ⚠️
internal/librarian/add.go	0.00%	1 Missing ⚠️
internal/librarian/fake.go	0.00%	1 Missing ⚠️
internal/librarian/rust/codec.go	0.00%	1 Missing ⚠️
internal/librarian/rust/update_manifest.go	0.00%	1 Missing ⚠️
... and 1 more

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3690   +/-   ##
=======================================
  Coverage   82.43%   82.44%           
=======================================
  Files         138      138           
  Lines       12657    12656    -1     
=======================================
  Hits        10434    10434           
+ Misses       1733     1732    -1     
  Partials      490      490           

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