Change POP3 error response detection

Instead of interpreting a "-" at the beginning of a line as
an error response, consider the absence of a "+" at the
beginning of a line as an error response.

This is what Thunderbird does.

http://hg.mozilla.org/releases/comm-esr24/file/55e96a433bd1/mailnews/local/src/nsPop3Protocol.cpp#l1177

The problem arises with godaddy servers spewing additional
lines of data upon login failure.  The login was being
interpreted as successful, and a STAT commanded was subsequently
being sent, resulting in a dialog saying 'Cannot connect to
server. (Invalid int: "auth_error:")'.

$ openssl s_client -quiet -crlf -connect pop.secureserver.net:995
...
+OK <24984.1394317012@pop.secureserver.net>
user testuser
+OK
pass testpass
testuser not found in the auth database
warning: auth_error: authorization failed (no such object)
-ERR authorization failed  Check your server settings.