Return better error when images fail to be fetched

[mihow]

Currently if an image is a local URL or any bad URL it returns a generic 500 error. A user data error (400) with details about the failing image would be much more helpful. There's also a chance more details are being returned with the 500 error but Antenna is not displaying them.

Example from the logs:

2025-08-09 01:26:21 [info     ] Downloading http://django:8000/media/example_captures/19/black-witch-moth-20231112123850-snapshot.jpeg to /tmp/ami_imagesbm3gmspn/black-witch-moth-20231112123850-snapshot.jpeg
INFO:     192.168.123.178:35684 - "POST /process HTTP/1.1" 500 Internal Server Error