Rename 'findPackageDesc' to 'tryFindPackageDesc'.

Make 'findPackageDesc' return an 'Either'.
'cabal repl': when there's no .cabal file, just start ghci.

Fixes #1554.
Merge pull request #1607 from 23Skidoo/repl-no-project

'cabal repl': when there's no .cabal file, just start ghci.