Replace some TResponseError with ResponseError

[joyfulmantis]

This a surgical edit to replace some of the TResponseErrors with ResponseErrors, making it easier to port HLS to the new release. This is one of the possible permanent solutions moving forward. Other possible options are:

• Keeping the TResponseMessage as is and creating another type in between TResponseMessage and ResponseMessage.
• Implementing these changes at the same time as also creating a new fully typed ResponseMessage.
• Refactoring HLS to use TResponseErrors (I feel it should be possible, but will need someone more familiar with Haskell type magic than I am).

[michaelpj]

Thanks, this looks fine to me!