Utilities/Sphinx: Avoid deprecated docutils error handling helpers

The `SafeString` and `ErrorString` classes are deprecated and will be
removed in Docutils 0.21 or later.  They help avoid encoding errors
during exception handling in Python <= 2.7, but these days we always
use Python 3 to build the documentation, at least during development,
when error cases are most likely to occur.

Fixes: #24039