bt2: make log functions clear error indicator

bt2: make log functions clear error indicator

In all call sites of logw_exception and loge_exception_append_cause, we
clear the error indicator afterwards (call PyErr_Clear).  Make these
functions clear the error indicator themselves instead, and rename them
accordingly.  This could help avoid forgetting to clear the indicator in
the future.

Change-Id: I22855c1bb4af0e5b85bafc3f1009589ee08c0564
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/2225
Reviewed-by: Francis Deslauriers <francis.deslauriers@efficios.com>