Include machinery backend error details in logs

[hemaa0325]

Summary

This change improves error reporting for machine translation backend failures.

Previously, when translation fetching failed, Weblate only reported a generic error message:

Could not fetch translations

This update extracts the actual backend error message and includes it in report_error() using extra_log.

Changes made

• Extract backend error message using get_error_message(exc)
• Pass detailed error message into report_error() through extra_log
• Reuse the extracted message when raising MachineTranslationError

Benefit

This helps administrators understand backend failures such as rate limits, invalid credentials, and temporary service outages.

[nijel]

The test assertion will have to be adjusted to match changed behavior.