bt2: use format_bt_error and format_bt_error_cause to generate _Error and _ErrorCause...

This makes it so that formatting bt2._Error as a string looks the same
way as what is printed by the CLI.  It also allows formatting of error
causes individually.

The __str__ methods generate string representations that do not include
terminal color control characters. If we want, we could later add a
"to_string" method that can optionally generate a string with terminal
color control characters.

Change-Id: I9e2c6db536a1bea46b07c5fe8bb13f702d8accce
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/2435
Tested-by: jenkins <jenkins@lttng.org>