improves the robustness of the imap reconnect feature which was
introduced in commit beae17a6da ("imap: auto-reconnects on connection
error").
If a connection error is emitted, the message list is cleared and a
corresponding error message is shown in the ui. Status bar is updated as
well. Upon reconnect, the directories and the message list will be
re-fetched (same behavior as the connect command).
Reconnect can be enabled and disabled with the connect and the
disconnect commands.
Signed-off-by: Koni Marti <koni.marti@gmail.com>