This PR fixes poor error reporting from Verso docstrings. Before, if the
Verso parser didn't consume the whole docstring, then Lean would try to
parse the closing -/ and fail; this would lead to backtracking and an
assumption that the docstring must be non-Verso, with only the non-Verso
commands like #guard_msgs as possibilities. Now, the input is always
consumed.
Closes#12118.