Improve error message for scalar types in refinements

[senier]

package Test is
   type Length is mod 2**16;
   type Address is mod 2**32;

   type Message is
      message
         Length : Length
            then Data
               with Length => 8 * Length;
         Data : Opaque;
      end message;

   for Message use (Data => Address)
      if Length = 17;                                                                                                                                                         
end Test;

Using a scalar type in a refinement, yields the following error:

Parsing test.rflx
Processing Test
test.rflx:13:29: parser: error: undefined type "Test.Address" in refinement of "Test.Message"

We should either support that or at least emit a better error message (stating that the type was found, but is not suitable).

[treiher]

The scalar type could be wrapped in a message to make such a refinement possible. As there is no concrete use case, I don't think it would be worth the effort to support this special case. We should just improve the error message.

[treiher]

The error message is improved in version 0.21.0.