stbt run: Dump pipeline-error debug from the right pipeline

If you define the environment variable `GST_DEBUG_DUMP_DOT_DIR` and a
GStreamer error occurs, `stbt run` will dump a Graphviz dot file with
debugging information (see 7b3eaf33). Previously we always dumped
debugging information for the *source* pipeline, even if the error came
from the *sink* pipeline.