Forward the error to the user via a KMessageBox

Otherwise i can't say the message that says

No s'ha pogut iniciar la sessió al servidor pop.mail.yahoo.com.\nCould not login to pop.mail.yahoo.com. The password may be wrong.\n\nThe server said: \"[AUTH] (#MBR1213) Please verify your account by going to https://login.yahoo.com\"

And i'm stuck not being able to download email.