Fix a few places that re-raise ValueError:
Fix a few places that re-raise ValueError:

Rather than throwing away the information in the original exception,
we will instead _add_ some information.

This should make it easier to see _why_ we're failing to parse an
iCalendar file.

Signed-off-by: Rok Garbas <91d1dd8615d34a96099572be3f37ee9127974eeb@garbas.si>

geier/icalendar,untitaker/icalendar,nylas/icalendar