core: Use one line error log instead stack trace.

Instead logging all the stack trace when failing to end a command after
task finishes, we will use one line error message,
that will indicate the error code and the error name got from VDSM.

Change-Id: I1907d94af939bbe6da6a2149428a97c5f33fe826
Signed-off-by: Maor Lipchuk <mlipchuk@redhat.com>
Bug-Url: https://bugzilla.redhat.com/908854