better error message for `outputSource` extra element

[mr-c]

outputs:
  many_files:
    type: File[]
    outputSource:
      source:
        - step1/array1
        - step2/array2
      linkMerge: merge_flattened

ERROR Tool definition failed validation:
result_chunker_subwf.cwl:2:1:  Object `result_chunker_subwf.cwl` is not valid because
                 tried `Workflow` but
result_chunker_subwf.cwl:17:1:   the `outputs` field is not valid because
result_chunker_subwf.cwl:18:3:    item is invalid because
result_chunker_subwf.cwl:20:5:     the `outputSource` field is not valid because
                     value is a CommentedMap, expected null or string or array
                     of <string>

I would suggest that when a single entry Map appears where no Map is valid, that the parser tries again to see if the contents of that solo value in the map otherwise parse; only for the purposes of giving a better error message

result_chunker_subwf.cwl:20:5: the outputSource field is not valid because value is a Map, expected null or string or array of . The outputSource field is valid if the source: on line 21 is removed.