Set cwd to `get_source_dir(filename)` when running ghc-mod.

Fixes error that ghc-mod can't find a library in an executable in the following setup:

    name: mypackage
    library
      hs-source-dirs: src
    executable
      hs-source-dirs: apps
Show cabal warnings in the panel.

Before we only highlighted warnings, but did not show their text.

Thanks @eis in #haskell.