- `@[fail] def foo = ...` succeeds if `foo` has some error.
- `@[fail "scope"] def foo = ...` succeeds if `foo` has some error
containing the word "scope" somewhere
- `@[fail] namespace foo { }` works too and the error must be anywhere
in the namespace
previously it backtracked too much, so instead of giving a useful
parse error, it just said "expected end of input" at the beginning of
the problem toplevel. which, if it's a namespace, could be way off.
