broaden exception handling on webcal refresh

when iterating through a calendar, recurrance events can throw an
exception if no instances of the recurrance are found.

this exception is of class `Exception` but the try/catch clause in the
webcal refresh loop only catches `BadRequest` exception.

this leads to the exception bubbling up and thus other calendar events
do not get processed by the event iterator.

this PR broadens the exception to handle both BadRequest and
NoInstanceFoundException so that the full webcal can be processed, even
if minor hiccups are processing on vobject

Signed-off-by: leith abdulla <online-nextcloud@eleith.com>