Add minor debug messages on connect and folder traversal

[bentolor]

Thanks for your little helper tool, @rgladwell

It didn't work at first for my Thunderbird ImportExportTools NG, so I added a few debug statements which eventually helped me to understand, that

• The connection works
• The directory tree is traversed
• File mbox files are skipped due to naming convention.

[rgladwell]

Thank you for the useful PR, and I'm glad you imap-upload useful.