Handle and show (friendly) errors to the user

[ahankinson]

UV version:

https://uv-v4.netlify.app

I'm submitting a:

• bug report => please fork one of these codesandbox examples with a repro of your issue and include a link to it below
• feature request => please use the user stories repo
• support request => Please do not submit support requests here, use stackoverflow

Current behavior:
If you pass in a URL to a manifest that returns a 404 error, UV should show some error to the user. Currently UV will print a message to the browser console, but will not show any indication of the problem to the user.

https://uv-v4.netlify.app/#?manifest=https://iiif.bodleian.ox.ac.uk/iiif/manifest/e32a277e-91e2-4a6d-8ba6-cc4bad23041.json

This is a purposefully misconstructed URL (it's missing a 0 at the end of the manifest UUID). Currently the viewer just shows a spinning circle, with no indication that loading failed.

In fact, any non-200 error (with the possible exception of routes subject to the IIIF auth api) should be handled and displayed to the user. Particularly 400, 404, and 5XX errors.

Expected behavior:
The viewer should show the user feedback if loading the manifest failed. The user should see a friendly error that indicated what went wrong. ("We're sorry, but that manifest could not be found." or something along those lines)

[demiankatz]

This overlaps with #162 but provides some helpful additional detail. I'll leave both issues open rather than closing one as a duplicate. Hopefully we can make some progress on this on our next bug-fixing push (though right now, community energy is focused on website updates, so it may take a little while to get back to it).