Content with 4xx status codes

[Neale Upstone]

Hi,

Using 0.3.0, I'm finding that my end users would like more information.  
I'd like to return them something other than a <T extends Base>, 
provided for by BaseResponseContext.

Perhaps it would be useful to have an ErrorResponseContext that could be 
used instead of EmptyResponseContext when returning things like "Bad 
Request"

Cheers,

Neale