#1 No error is shown when nonexistent mailbox selected

[Nodir Yuldashev]

For IMAP connection when in settings a non-existent mailbox is selected no error is shown to user.

How to reproduce:
1. have an IMAP account
2. in Accounts window in "IMAP4 mailboxes separated by comas:" put a non-existent mailbox name
3. try to retrieve mails from that account -> no error should be shown.