[sylpheed:34789] Error message on wrong IMAP password

[sopra-it at gmx.net]

Hi

If entering a wrong password when accessing an IMAP folder, the error message
says something like "Could not connect to the server". 
As this is a little bit misleading, I'd suggest to change it for something more
meaningful like "Wrong IMAP password" and offer to retry.

Sorry for supplying a patch, but my coding skills are limited...