Adjusted error handling on one-sided sync

I noticed that vDirSyncer sometimes has issues with certain CalDAV calendar events; maybe they are not fully CalDAV compliant, or SabreDAV does not support some features, for example I often get

• Unknown media type
• COUNT IN RRULE not a positive integer or
• This recurrence rule does not generate any valid instances.

If the source calendar is configured as read_only then it would be highly beneficial to create the target event even if there are fragments not supported, for example it creates an event but skips the "unknown media type" info.

Behaviour today:

• All events with whatever error are skipped and do not show up
• vDirSyncer will re-try on each and every sync again to process the VEVENT, with always the same error

Expected behaviour:

• Events with errors are still created, but with stripped content, and eventually a remark in the notes field.

Not syncing events then at all is a bigger issue as appointments get lost, even if otherwise only an unsupported attachment would be deleted.

Start by tracing the one-sided sync path and the handling of VEVENT parsing errors for read-only sources. Review how errors currently cause events to be skipped and check existing tests around synchronization and malformed calendar data. Done means supported portions of an event are created, unsupported content is omitted, and the error is recorded in the target notes without repeated unchanged failures.