Throw a nice exception if an invalid js-style is used

[thecapdan]

e.g. If we have an app has .js-style file that contains the old 'node,js' notation, then we should throw an exception, and maybe list the current valid styles that can be used.

[dchambers]

This isn't quite as easy-picking as you might otherwise expect since we need a way to ask each plug-in what styles they recognize, then only throw an exception if none of the plug-ins recognize it.

[andy-berry-dev]

@dchambers Agreed, but this issue is just to throw the exception so it is reasonably straightforward. If we don't already we need a separate issue to make the .js-style pluggable.

[andy-berry-dev]

I've raised #929 to track making JS styles pluggable.

[dchambers]

Just to be clear, the '.js-style' file is currently plug-able. So, in effect, you're proposing creating technical debt as a result of this issue that will mean it isn't, that we'll then fix at a later date in #929? If so, is there a particular reason why this issue is worthy of creating technical debt, since it's not something we normally do?