stbt.Display.on_warning: Don't check for OpenCV errors from GStreamer element

We no longer use OpenCV from the GStreamer element (in fact we don't use
the GStreamer elements "stbt-motiondetect" and "stbt-templatematch" at
all).
squash? stbt.py: Run GLib main loop continuously

Communicate errors from the GLib-main-loop thread to the main
application thread running the user's script.

This needs some tests.
squash! REWORD (add reference to commit removing gst/): Remove tests for GStreamer motiondetect element

also templatematch element