lttng: mention argument number on unknown action / condition name

lttng: mention argument number on unknown action / condition name

When a unrecognized condition or action name is given, the error message
does not contain the part that mentions the argument index, like
argument parsing error messages have:

    $ lttng add-trigger  --action notify  --condition pouet
    Error: Unknown condition name 'pouet'

I think it would be useful to have it, it can help when you have really
long command lines (something possible with add-trigger).  The result
is:

    $ lttng add-trigger  --action notify  --condition pouet
    Error: While parsing argument #4 (`--condition`): Unknown condition name 'pouet'

    $ lttng add-trigger  --action notify  --condition=pouet
    Error: While parsing argument #4 (`--condition=pouet`): Unknown condition name 'pouet'

    $ lttng add-trigger --condition event-rule-matches --action pouet
    Error: While parsing argument #4 (`--action`): Unknown action name: pouet

    $ lttng add-trigger --condition event-rule-matches --action=pouet
    Error: While parsing argument #4 (`--action=pouet`): Unknown action name: pouet

Change-Id: Ic1a00cffa87ea7b3569b24c7417a00d7a52555f2
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>