Error handling

[hc-nolan]

Error handling is currently pretty abysmal. When something goes wrong the user doesn't know why until investigating the app logs.

Various errors to improve

• Integrity errors on insert (release, label, artist)
  • Make sure that if db.util.handle_submit_data() raises IntegrityError, a page is presented to the user that tells them which field caused the integrity error
• Unsupported image type in URL
  • e.g. go to edit an artist image and try https://skierscribbler.com/10748/opinion/gang-starrs-influence-on-hip-hop/