Rewrite incomplete pattern matches with stub

[yaitskov]

GHC doesn't generate stack trace in case of incomplete pattern match.

Up to date
App/DepB.hs:7:1-14: Non-exhaustive patterns in function fB

fB :: HasCallStack => Int -> Int
fB 1 = 1 

Would be cool to append default case to fB

fB :: HasCallStack => Int -> Int
fB 1 = 1 
fB o = error $ "DepB.fB is not implemented for " ++ show o

Then output is much more informative:

CallStack (from HasCallStack):
  error, called at ../App/DepB.hs:8:8 in main:DepB
  fB, called at ../App/DepA.hs:7:8 in main:.DepA
  fA, called at ../App/Main.hs:108:23 in main:Spec

[bitc]

I think it would be better to add this feature directly to GHC, so that everyone gets a stack trace, and not just those of us who use this plugin.

And while we are at it, in addition to incomplete pattern match, we should also give stack trace to the error Missing field in record construction