Don't require connection to pypi

[sbourkeostk]

Allow offline use.

Currently the program requires a http request to pypi to check for newer versions. In the case where that connection is not possible Eg. no network, firewall, etc., backup-warden cannot be used.

This PR does two things:

1. Adds an option to disable the update check
2. Handles the exception when the request fails and log a warning

[charles-001]

Hi @sbourkeostk - nice catch! Thank you for your contribution