EWebDAVDiscover can report failure on success and hide collections

Two issues:
a) when the server returns success on the given URL PROPFIND, but
   doesn't support /.well-known/ addresses, then the try on the
   /.well-known/ could overwrite the success of the given URL PROPFIND,
   causing return failure without setting the GError;
b) when the given URL is a collection of the type the output is filtered
   for, the URL could be skipped from consideration and not included
   in the output discovered resources.

Reported downstream as a gnome-calendar crash:
https://bugzilla.redhat.com/show_bug.cgi?id=1287073