After gst-validate-1.0 notices the first expected error on the bus, it will
queue the completion of that action while the pipeline is still processing the
error and getting to the basesrc for the second error.
Mark the second error as 'sometimes=true', as it's not critical to the test.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/9060>