fix: handle exceptions without 'detail' attribute in error handler

[rasadov]

Summary

Fixes #253

Reopening of #254 - When Redis (or another storage backend) is unavailable, slowapi's error handler tries to access exc.detail on a ConnectionError exception, which doesn't have this attribute. This causes a secondary AttributeError that masks the original connection failure and crashes the application.

Changes

• Modified _rate_limit_exceeded_handler to accept Exception instead of RateLimitExceeded
• Used getattr(exc, 'detail', "Rate limit exceeded") to safely access the detail attribute
• Added fallback logic: use default message for RateLimitExceeded, or str(exc) for other exceptions
• Added comprehensive tests for both RateLimitExceeded and exceptions without detail scenarios

Root Cause

The error handler in slowapi/extension.py directly accessed exc.detail without checking if the attribute exists. When a ConnectionError (or other exception types) is raised during rate limit checking, this assumption fails because ConnectionError doesn't have a detail attribute.

Testing

• Added test test_rate_limit_exceeded_handler_with_detail to verify normal RateLimitExceeded exceptions still work correctly
• Added test test_rate_limit_exceeded_handler_without_detail to verify exceptions without a detail attribute are handled gracefully

Backward Compatibility

✅ No breaking changes - existing code using RateLimitExceeded continues to work as before
✅ Improved error handling for edge cases (connection failures)

Closes #253