compose: Show an error dialog when a message edit fails, not just feedback in message list

[chrisbobbe]

When you tap "Save" to edit an already-sent message (#126) and the edit-message request fails, we show "EDIT NOT SAVED" in the message list:

That feedback doesn't offer any details about why the edit failed. This issue is to show some details, in an error dialog that appears when the request fails.

Prompted by noticing that we're planning to show a dialog on failed-send (of a new message) for #1441, and thinking we'd like to be consistent with that.