Feat improve error message

[sgao640]

@livefire2015
@tendant

[sgao640]

Summary

Improves error handling across the HTTP API by implementing structured, user-friendly error messages while preserving detailed internal logging.

Key Changes:

• Handler updates: All error responses now use ToErrorMessage() to return formatted, caller-friendly messages instead of raw err.Error() output
• Enhanced error types: Added ErrorMessage() and HTTPStatus() methods to ContentError, ObjectError, and StorageError for consistent error formatting
• New sentinel errors: Added ErrMaxDerivationDepth, ErrNoStorageBackend, ErrNoObjectsFound, ErrNoUploadedObjects for better error categorization
• Service layer improvements: Replaced generic fmt.Errorf with typed errors (ContentError, ObjectError) for proper error wrapping
• Documentation: Added comprehensive ERROR_HANDLING.md guide

Benefits:

• ✅ Better security - no longer exposes internal implementation details in API responses
• ✅ Consistent error formatting across all endpoints
• ✅ Appropriate HTTP status codes based on error type
• ✅ Maintains detailed logging for debugging while showing clean messages to API consumers

Example:

Before: "parent content not found: sql: no rows in result set"
After:  "create_derived: content not found"