Don't return 500 based on malformed URL or user input in general

[jordanade]

I don't like that any bad image URL (say, out of range or incorrectly specified parameters) creates 500 errors. AWS, for example, sends me warnings that my instance is "degraded". There's nothing I can do to fix it!

[jordanade]

I feel like bad requests should return a 4xx code, like 422.

[sdebacker]

Hello, we are open to pull requests with tests.