getCurrentDirectory should raise better error string when cwd doesn't exist

[thomie]

@ezyang reports in https://ghc.haskell.org/trac/ghc/ticket/11280:

Run this script in a directory which no longer exists:

import System.Directory
main = getCurrentDirectory >>= putStrLn

Currently the error looks like:

ezyang@sabre:~/test$ ./test
bash: ./test: No such file or directory

It would be nicer to give a better message in this case.

Originally reported: ​haskell/cabal#2902

[ezyang]

OK, this is not actually a bug with directory, and also there's been a commit which improves it in any case.