py-common: make bt_py_common_format_exception accept an arbitrary exception

py-common: make bt_py_common_format_exception accept an arbitrary exception

A following patch will need to format each exception of the exception
chain independently.  It will therefore need a function like
bt_py_common_format_exception, but that:

- takes the exception to format as a parameter, rather than relying on
  the Python error indicator
- has an option for formatting just that exception, not following the
  causes chain

This patch changes bt_py_common_format_exception to meet these
requirements.  The behavior of the old bt_py_common_format_exception is
now offered by bt_py_common_format_current_exception, so the existing
callers are updated to use that one.

Change-Id: I7bbb442bc1dc363e5441b3b11ed6f0c6e213ba79
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1737
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>