Make sure imaplib exception argument is a string before using split() (#5387)

Summary:
Looks like imaplib in some conditions can throw exceptions without the message or
the message is not a string. We'll cast to string before using split() to prevent from
"AttributeError: 'int' object has no attribute 'split'" errors.

Reviewers: PyKolab Developers, vanmeeuwen

Reviewed By: PyKolab Developers, vanmeeuwen

Differential Revision: https://git.kolab.org/D135