scripts/device-crash-test: hide tracebacks for QMP connect errors

Generally, the traceback for a connection failure is uninteresting and
all we need to know is that the connection attempt failed.

Reduce the verbosity in these cases, except when debugging.

Signed-off-by: John Snow <jsnow@redhat.com>
Reported-by: Thomas Huth <thuth@redhat.com>
Tested-by: Thomas Huth <thuth@redhat.com>
Message-id: 20211111143719.2162525-6-jsnow@redhat.com
Signed-off-by: John Snow <jsnow@redhat.com>