GUI error should be displayed when saving event fails

### Problem

I found this while confirming #726. When that event fails to save, we do not propagate any information to the user, except that the event they tried to save simply doesn't appear in the calendar. We also don't provide them any opportunity to recover their lost data.

### Proposal

We should provide some sort of graphical warning that saving the event failed.

I'm not sure what form this should take. The simplest would be to present something like an infobar. However, if we want to provide users a way to recover the lost event, we should add something else. I don't recall the current saving architecture, so would it be possible to check for a failure before exiting the editor dialog? That way we could display the error there and provide an opportunity for the user to fix the issue before the editor ever closes.

### Prior Art (Optional)

_No response_