Better error message.
Be more strict when parsing 'cabal.config'.

In rare cases (when 'cabal-dev -s .' was used) its 'cabal.config' could conflict
with our 'cabal.config'.
Refuse to create a sandbox if there's already a cabal-dev sandbox.

Also improve a message that gets printed when we find a cabal.config created by
cabal-dev.
80-col violations.