It was an crash before, if we have stored CMIS url with repository
that does not exist. Session was created, but with empty repo.
Now this nuance is checked and slightly improved error reporting for
cmis.

Change-Id: I447ec767fd735829f8a507733552b26e05cba441
Reviewed-on: https://gerrit.libreoffice.org/17224Reviewed-by: Thorsten Behrens <Thorsten.Behrens@CIB.de>
Tested-by: Thorsten Behrens <Thorsten.Behrens@CIB.de>