So we are now looking into the possibility of adding your own error
messages to GHC as our project. We have about 8 weeks to implement,
either all or some proof of concept. As such we are looking for
comments, ideas and suggestions. Is this doable in GHC?, or what is
needed.

For related work, see the publications of Bastiaan Heeren, esp. his PhD thesis. Search the following page for "error:"

  http://people.cs.uu.nl/bastiaan/

Regards,
Sean