runghc should give an error when the path to ghc is invalid
runghc has a -f option which allows to specify the path to GHC. However, it uses
rawSystem and if the path is incorrect, we only get a nonzero exit code. For example, attempting to use "ghc -fdefer-type-errors X.hs" gives absolutely no output.
Attached patch uses
findExecutable and gives an error message if the executable is not found. This is not the ideal solution, but I think it's better than the current state.