restapi: Do not send ticket back in case of operation error

Do not return a ticket using REST API if operation fails,
e.g. user doesn't have permission to VM

Change-Id: I8bc4d0c6ed48005978d333d28c7bec66d076c511
Bug-Url: https://bugzilla.redhat.com/867920
Signed-off-by: Libor Spevak <lspevak@redhat.com>