Make duplicate param name errors mark the actual duplicate parameter

Before:

    coffee> foo = (bar, baz, bar, qux) ->
    repl:1:7: error: multiple parameters named 'bar'
    foo = (bar, baz, bar, qux) ->
          ^^^^^^^^^^^^^^^^^^^^^^^

Now:

    coffee> foo = (bar, baz, bar, qux) ->
    repl:1:18: error: multiple parameters named 'bar'
    foo = (bar, baz, bar, qux) ->
                     ^^^

Also works with destructuring parameters and what have you.