[Dovecot] NONEXISTENT handling

[Michael M Slusarz]

As of 2.0.12, looks like NONEXISTENT response code isn't working  
properly again.  Previously brought up issues relating to this here:  
http://markmail.org/message/jxbgdfv26pws66og

With a nonexistent mailbox "FOO", I see this:
1 DELETE "FOO"
1 NO Mailbox doesn't exist: FOO
// Should be: 1 NO [NONEXISTENT] Mailbox doesn't exist: FOO

2 RENAME "FOO" "BAR"
2 NO Mailbox doesn't exist: FOO
// Should be: 2 NO [NONEXISTENT] Mailbox doesn't exist: FOO

michael