improving error messages

[Simon Peyton-Jones]

| as you may remember, in GHC survey awkward error messages was named as
| one of most serious GHC drawbacks. i propose to start collecting
| examples of bad error messages together with what we want to see in
| these cases. as first contribution, i've added this text as
| http://hackage.haskell.org/trac/ghc/ticket/956

Thank you!  It'll be very useful to have such a collection.  

Simon