enhance error messaging

[Garfonso]

Hi,

I use the adapter for quite some time now and it works ok. Thanks for that 😄

But I must have at least one corrupted calendar entry somewhere, because I get this error message regularly:

error: calendar.0 (613799) VCalendarError: <constructor> <parse> Unknown RRULE property 'Europe'

It would help if the error message could give a hint towards the erroneous entry, if possible (for example the UID or filename or something like that).