Make symbolic references diagnostic less cryptic

Make symbolic references diagnostic less cryptic

If somebody has accidentally used a symbolic reference with strict enabled and
are looking for diagnostics then it's likely they didn't intend to use a
symbolic reference at all. A beginner may not even know what a symbolic
reference is, or even a reference.

So explain the error in terms of what the user has done, not what sort of
references are allowed. Provide a simple example of how this error can occur,
so even those who don't know about references have a chance of spotting and
fixing their mistake.