Saving partial state resulted in 500 error using git backend

[dev-dull]

Currently unclear how to recreate the issue, but a terraform job was canceled (via ^c) during apply, PyTerraBackTYL responded with a 500 error while saving partial state. The canceled job gracefully exited and the resulting state was successfully saved. No error was generated in the log.

To help catch the exact cause, error messages should be logged in the Flask internal-server-error (500) error handler.