Make the frontend handle a server error (500) on batch diffs gracefully
Summary
When the batch diffs trip and spit out a 500, the FE just sits there without any error.
This at least happens in the case described in:
Just ran into this over here. When opening the diff, the spinner just spins indefinitely with no indication that anything went wrong:
At the very least we could improve this with a "Something went wrong fetching the diff" on the frontend
🤷
Steps to reproduce
Example Project
What is the current bug behavior?
No diffs show up and no error message is shown.
Heads up: please check Show one file at a time and Show whitespace changes combinations.
What is the expected correct behavior?
If there's an error, properly inform the user.