Introduce XFAILs that assert failure

Currently our XFAILs are open ended; we do not check that it actually
fails, so we have no easy way of knowing that a previously-failing test
starts passing. Introduce a new enum that we return from overrides to
give us more flexibility here, including the ability to assert that
expected failures happen.

This includes refactoring of `precision.rs` to be more accurate about
where errors come from, if possible.

Fixes: rust-lang#455