Error feedback UX when account storage things fail

This MR adds minimal error feedback for the user when something goes wrong while loading or otherwise manipulating account storage. Feedback consists of a simple inline error message being displayed on top of the accounts overview list, stating something has gone wrong.

The most likely scenario for triggering the message is if the user "unlocked" the accounts with the wrong password. A less likely but still plausible scenario would be corrupted file input while loading accounts. Finally, something unexpected could happen tripping the sanity checks: that would imply a bug in Keysmith application logic (but at least now it is visible to the user and we might get bug reports if that happens).

This MR builds on top of !36 (merged) but is otherwise independent from it. However, it does require some of the changes in !35 (merged).