Fix: fix hard stop when facing 404 resources on the server

[mgumz]

Everyone's darling Google-Caldav server might reference resources which, in the end, do not exist:

debug:   <ns0:href>/caldav/v2/[email protected]/events/[email protected]</ns0:href>
debug:   <ns0:status>HTTP/1.1 404 Not Found</ns0:status>

This situation leads to an unconditional hard stop of processing all the other, valid and available calendar entries:

error: Unknown error occurred for my_sync/[email protected]: /caldav/v2/[email protected]/events/[email protected]
error: Use `-vdebug` to see the full traceback.

Instead of a hard stop, this PR changes the behaviour of vdirsyncer to just print a warning and continue.