I think there are four groups of errors, based on what the user can do about it:

1. intermittent errors in the AJAX request (network errors, server being busy)

2. bugs

3. session timeout - user needs to log in again

4. localstorage errors (strict security settings or quota exceeded, or even no localstorage support at all)

1. and 2. could be covered by something like "there was an error, please retry and report it if it persists". 3. is very rare and will cease to be at the next page load so it's probably OK to ignore it.

I'm not sure about 4 - that's something that needs user intervention, and a source of complaints (there have been reports that it is impossible for anons to disable Media Viewer, which I suppose is caused by this). Localstorage settings are a poweruser feature though, not sure how to communicate about them.