cli: exit with error if a plugin fails to load

The CLI is currently permissive when plugins fails to load, it will
display an error but carry on.  To make it easier for users to catch
configuration and programming problems, this patch changes that be
strict.  If a plugin fails to load (for example, Python syntax error),
the CLI stops and the error is reported to the user.

If this ever becomes a problem (a user would like to carry on even
though some plugin fails to load), we can add a switch like
`--ignore-plugin-loading-error` to have a permissive mode.

Change-Id: Ic89d4e044ecdc34a6e169309479e17bb947ae637
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1937
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>