lib: remove LOADING_ERROR status

Remove the LOADING_ERROR status, use ERROR instead.  It's not so useful
to return a separate kind of error, now that we have a way to
communicate detailed error causes.

Change-Id: I45682b12748fdf8a8466211323dd8563f01d423b
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1754
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Tested-by: jenkins <jenkins@lttng.org>