virsh: Report error when explicit connection fails

virsh: Report error when explicit connection fails

Commit 0c56d9431839 forgot to return false in the cmdConnect command
after the clean up made there.

Before (assuming you don't have uri alias for 'asdf'):
  $ virsh connect asdf
  error: failed to connect to the hypervisor

  $ echo $?
  0

After (with the same assumption):
  $ virsh connect asdf
  error: failed to connect to the hypervisor
  error: no connection driver available for asdf

  $ echo $?
  1

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1356461

Signed-off-by: Martin Kletzander <mkletzan@redhat.com>