notmuch-show.el: handle the case where icalendar-import-buffer returns nil

notmuch-show.el: handle the case where icalendar-import-buffer returns nil

icalendar-import-buffer can fail by an error signal (which have been
witnessed) but according to its docstring it can also return nil
when failing (it returns t when succeeding).

Now that the error is caught by the caller of notmuch-show-inset-part-*
functions in case icalendar-import-buffer returns nil an explicit
error is signaled and unwind-protect takes care of deleting the
temporary file (just in case, it is usually not written to the fs yet).