errorformat file path relative to dune-workspace root

[ygrek]

in multi-project dune workspace with nested directories error messages from compiler are relative to root of dune-workspace, not to the current working directory. It would be nice if ocaml.vim could resolve them correctly.
e.g. dune-workspace is in ~/project and my cwd is ~/project/a/b then error will mention file "a/b/c.ml" and vim "go to error" does not work..