Create a GstException to wrap a Gst.GError.
Create a base GstPipelineTask class.
Use it in Checksum and TRM tasks.
Raise and don't proceed to call .paused() when a GstError happens.
Should help debug https://bugs.launchpad.net/bugs/735053
* morituri/test/test_common_checksum.py:
Adapt test.