tests: add expected error strings cli/convert/test_convert_args

tests: add expected error strings cli/convert/test_convert_args

It looks like this test was meant to optionally check the error strings,
the in the "failure" test cases, but it's not used at the moment.  This
patch adds an error string to check for each use of
test_bt_convert_fails.

Change-Id: I253743fdd174be5ba09c0b07f4fd1bc4dbf767da
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/3147
Tested-by: jenkins <jenkins@lttng.org>