Ensure directory exists before searching for a .cabal file

[ivan-m]

As requested by @gracjan in #506, this makes haskell-cabal-find-file check that the provided directory actually exists before using it to search for a .cabal file.

[ivan-m]

Actually, this might not actually work: if the provided directory is nil, then default-directory is used... which will also be nil as that's the directory of the buffer.

So let me do this another way then.