"Save draft" returned early without saving when the identities hadn't
loaded (offline when the composer opened) or the account had no Drafts
folder, and the close dialog then closed the composer as if the save had
worked, dropping the text.
The close dialog now offers only Cancel and Discard in that case and
says why the draft can't be saved; the save path refuses to close too.
The close guard moves from a bare `beforeRemove` listener to
`usePreventRemove`, which also stops the iOS modal swipe-down that the
listener could not prevent.
Linus Rath
92b42e4a4f