'cabal run': print a message before actually running the program.

Fixes #1898.
'bootstrap.sh': s/GZIP/GZIP_PROGRAM/.

Fixes #1902.
An OOM condition can apparently also show up as 'ExitFailure (-9)'.

See https://github.com/haskell/cabal/issues/1522#issuecomment-44381178
Merge 1.20