This PR fixes a bad error message due to elaborating partial syntax with
Verso docstrings.
When elaborating partial syntax, the elaborator sometimes attempts to
add a docstring for a declaration that it didn't parse a name for. The
name defaults to anonymous, but inserting the docs for the anonymous
name throws a panic about being on the wrong async branch.
With this change, the reported error is the expected parser error
instead, which is much friendlier.
