Handle Git Command Failures Gracefully to Avoid HTTP 500 Errors

[ouyangde]

Description
When using the jupyterhub-git extension, encountering an error with a Git command results in an HTTP 500 response. This is not ideal because HTTP 500 indicates a server-side error, whereas a Git command failure is a common occurrence and should be handled more gracefully.

Reproduce
Go to JupyterHub with the jupyterhub-git extension enabled.
Perform an action that triggers a Git command (e.g., commit, pull, push).
Ensure the Git command fails (e.g., by causing a merge conflict or using incorrect credentials).
Observe that the server returns an HTTP 500 error.
Expected behavior
The expected behavior is that when a Git command fails, the extension should handle the error gracefully and return an appropriate HTTP status code along with a descriptive error message. An HTTP 4xx series code (e.g., 400 Bad Request or 409 Conflict) would be more appropriate for client-side errors like these.

Context
Python package version:
Extension version:
Git version:
Operating System and its version:
Command Line Output
Web Browser Output
Additional Context
In many cases, Git commands can fail due to various reasons such as merge conflicts, incorrect credentials, or network issues. These are expected scenarios and should not cause the server to respond with an HTTP 500 error, which indicates an unexpected server-side failure. Properly handling these errors and returning a more appropriate status code would improve user experience and make debugging easier.