pythonplugin: Clean up error handling a bit
pythonplugin: Clean up error handling a bit

Don't g_error() but only g_critical() when things go wrong and return FALSE.
g_error() would kill the application immediately.

Also check if we can actually get gi.repository.Gst before using it.

pexip/gst-python,pexip/gst-python,GStreamer/gst-python,GStreamer/gst-python,GStreamer/gst-python,pexip/gst-python