map the LessError object to a real Error instance for consumption by gulp-util.PluginError.

LESS errors that trickle down through RECESS are an instance of a custom object; ```LessError```. Because ```PluginError``` expects an instance of ```Error``` in it's constructor, map LessError to Error and pass that real ```Error``` instance to ```PluginError```.

Verified that properties are retained after the mapping within ```realError``` and after it's passed to ```PluginError```.
But it's so much less pretty :) I'll update it.