Fix for error message around duplicate properties/descriptors

[nsaunders]

Description

This fixes the content of the error message that arises from duplicate properties or descriptors within a single ruleset.

For example, given this ruleset...

universal ? Rule.do
  width := nil
  width := nil -- Note the duplicate `width` property.

...the error currently reads as...

  Could not match type
           
    ( ... )
           
  with type
                          
    ( width :: CommonKeyword
    ...                   
    )  

After this change, it will read as...

  Could not match type
           
    ( ... )
           
  with type
                          
    ( width :: Measure Nil
    ...                   
    )  

...reflecting the real type of the property value.

Design considerations

N/A

Future plans

N/A

References

N/A

Code change checklist

• Any new or updated functionality includes corresponding unit test coverage.
• I have verified code formatting, run the unit tests, and checked for any changes in the examples.
• I have added an entry to the Unreleased section of the CHANGELOG.