restapi: Malformed GUID - fail gracefully

Until now, the http response would contain an unclear exception.
After this change, error message will be something like:

  "Invalid UUID string: SOME_@#%$@_BAD_^@$^_UUID"

Change-Id: Icf2d4db9b5e30a86b5eb2bb3071612a48867a646
Bug-Url: https://bugzilla.redhat.com/902073
Signed-off-by: Ori Liel <oliel@redhat.com>