Handle missing userfiles more gracefully

[snipe]

Previously, if an uploaded user file (not avatar, but uploaded via the "Files" tab on the user page) was missing from the server, the entire user page would crash. This change handles it a bit more gracefully and does not 500.

While this scenario shouldn't happen, it could, if a file was manually deleted, or if some files were lost.

Fixes RB-16844.

[what-the-diff]

• Added a new translation string for file_not_found
• Updated the view to show an icon if the file is not found, and added some logic around checking whether or not it's an image before displaying anything at all.

[marcusmoore]

Without actually testing myself (😅) this looks good to me 😄

[uberbrady]

This seems just a bit cleaner; looks good to me. Nice fix. I would've probably gotten lazy around the 'file not found' stuff, your way is going to be nice and clean.