Display more useful CalDAV errors

[raxod502]

CalDAV compliant servers may reject requests for failing standard preconditions. For example, here is a 403 response I received from Fastmail:

debug: <?xml version="1.0" encoding="utf-8"?>
debug: <D:error xmlns:D="DAV:" xmlns:C="urn:ietf:params:xml:ns:caldav">
debug:   <C:unique-scheduling-object-resource>
debug:     <D:href>/dav/calendars/user/redacted@fastmail.com/e3c5f94b-3c17-4b77-b22f-eff377305ad7/d765192e-52b7-4981-bbe4-67d3134d5abc.ics</D:href>
debug:   </C:unique-scheduling-object-resource>
debug: </D:error>

The precondition is documented at https://icalendar.org/CalDAV-Scheduling-RFC-6638/3-2-4-1-caldav-unique-scheduling-object-resource-precondition.html.

Currently, vdirsyncer does not make any attempt to interpret errors it receives from the server. In fact, prior to #1187, it does not even log them.

However, if a server returns an error message in a spec-compliant form, it seems like vdirsyncer should report that directly to the user, since the meaning is well-defined and understood.

[WhyNotHugo]

I agree in principle, but current vdirsyncer is mostly in maintenance mode with development efforts currently focusing on pimsync.

[raxod502]

Cool. Would you recommend that current users migrate their workloads to pimsync, or is development still at a stage where you would not recommend regular use?

[WhyNotHugo]

Aside from support for Google's flavour of CalDAV, a few other feature are still missing in pimsync. If you don't need any of those features, then I'd recommend using it instead.