plugins: log failure to load plugin as a warning

In my experience writing a Python plugin, it is really helpful to see
when it fails to load, with a traceback of the failure.  We currently
output this failure at the "info" level, which is not shown by default.
Even if I set the log level to info, that failure is lost in a sea of
messages.

So, even if fail_on_load_error is false, a plugin that fails to load
is a bit suspicious and is warn-worthy, in my opinion.

Change-Id: I82844aaa62d33dc0ad1059d515ae1581356f4fe9
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1706
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>