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.

With the new enum, it is also possible to specify ULP via return value
rather than passing a `&mut u32` parameter.

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

Fixes: #455

Author: tgross35