Errors helper should log condition where no plugins are properly configured on command-line

[michaelcfanning]

Certain tools that require plug-in specification error out when these arguments are malformed, with the result that users receive a generic 'no rules loaded' error.

A typical cause is a failure to provide the arg prefix, so that the path to the plug in is evaluated instead as a positional argument (e.g., a path to a potential scan target).