Tweak `expand_incomplete_parse` warning.

By using `token_descr`, as is done for many other errors, we can get
slightly better descriptions in error messages, e.g.
"macro expansion ignores token `let` and any following" becomes
"macro expansion ignores keyword `let` and any tokens following".

This will be more important once invisible delimiters start being
mentioned in error messages -- without this commit, that leads to error
messages such as "error at ``" because invisible delimiters are
pretty printed as an empty string.

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