The parser has some recovery heuristics but they will not always achieve
the best result. To prevent unsuccessful recovery from causing a cascade
of confusing follow-on errors, we'll track in the parser whether recovery
has been attempted and then the specific sub-parsers can use this to
skip certain "unexpected token type" errors in recovery situations,
assuming instead that an earlier error will cover the problem and that
we just want to bail out quickly.
The scanner produces complicated sequences for quoted strings due to the
template language, but sometimes we just want a simple string with no
interpolations.
