hadrian/ghci.sh assumes a cabal-based build

As a newcomer, I was excited to see hadrian/build.stack.sh. It worked flawlessly to build me a stage2 compiler.

Since I didn't see a hadrian/ghci.stack.sh, I assumed that the hadrian/ghci.sh script would work for me. It doesn't, as it assumes you built ghc via cabal.

An explicit error message saying "ghci only works for cabal based builds" would have saved me several hours of frustration. Better yet, a ghci.sh that works for stack would have tickled me pink!