Enhancement: Improve error handling for reviewer assignment

[Philipp0205]

Description

The error handling when adding reviewers needs improvement. Currently, when attempting invalid operations (like adding yourself as a reviewer when you're the PR creator), the user experiences poor UX with duplicate error popups.

Current Behavior

When trying to add yourself as a reviewer (when you're also the PR creator):

• Two error popups are displayed (duplicate error reporting)
• Error message shown:

  Failed to add yourself as reviewer
  GitHub API request failed: HTTP 422 - {"message":"Review cannot be requested from pull request author.","documentation_url":"https://docs.github.com/rest/pulls/review-requests#request-reviewers-for-a-pull-request","status":"422"}
  

Proposed Improvements

1. Client-Side Validation

• Validate before making API calls that the user is not trying to add themselves as a reviewer
• Check if the reviewer username matches the PR author
• Show a single, user-friendly error message without making the API call

2. Consolidated Error Handling

• Eliminate duplicate error popups
• Show only one error dialog per failed operation
• Review error handling flow to prevent cascading errors

3. User-Friendly Error Messages

• Transform API error responses into clearer, actionable messages
• Example: "You cannot add yourself as a reviewer on your own pull request"
• Remove technical details (HTTP codes, JSON responses) from user-facing messages
• Optionally: Log technical details for debugging purposes

4. Additional Validations to Consider

• Check if reviewer is already assigned before attempting to add
• Validate that the user exists and has appropriate permissions
• Handle other common API error scenarios gracefully

Benefits

• Better user experience with clear, actionable error messages
• Reduced API calls by validating before requests
• Single, consistent error notification instead of duplicates
• Less confusion for users

Technical Details

• Location: Reviewer assignment logic (likely in internal.ui package)
• Related: GitHub API error response handling
• Error code: HTTP 422 from GitHub API