Improve documentation/Readme

As per https://github.com/gulpjs/plugin-error/issues/8#issuecomment-358121678

> I think that a better solution could be to update the readme to explain why you should use plugin-error: "Error handling for Vinyl plugins." is a bit vague. For example:
>
> * Use this plugin so your plugin is formatted the same way as the others (consistency of the formatted message)
> * Use this plugin so consumers of your plugin can easily catch errors and filter them by plugin name (handle the errors programmatically)
> * Both

I think other things can be improved but nothing comes to mind immediately. This thread can be a grabbag for improvement suggestions.