Avoid throwing an error in initializeFlagsWithCradleWithMessage

[fendor]

This is mainly for the usage in haskell-ide-engine which dies a horrible death if the cabal file is invalid.

In particular, I would like to change:

initializeFlagsWithCradleWithMessage ::
  GhcMonad m
  => Maybe G.Messager
  -> FilePath -- The file we are loading it because of
  -> Cradle
  -> m ()
initializeFlagsWithCradleWithMessage msg fp cradle = do
    compOpts <- liftIO $ getCompilerOptions fp cradle
    case compOpts of
      Left err -> liftIO $ throwIO err
      Right opts -> initSessionWithMessage msg opts

to

initializeFlagsWithCradleWithMessage ::
  GhcMonad m
  => Maybe G.Messager
  -> FilePath -- The file we are loading it because of
  -> Cradle
  -> m (Maybe CradleError)
initializeFlagsWithCradleWithMessage msg fp cradle = do
    compOpts <- liftIO $ getCompilerOptions fp cradle
    case compOpts of
      Left err -> return $ Just err  
      Right opts -> initSessionWithMessage msg opts >> return Nothing

In the hope that CradleError will be promoted to an even more descriptive error type.

[fendor]

Due to an outdated remote, I totally overlooked the already existing changes! My bad!