Fix Span#setStatus

[jack-berg]

As reported by @laurit, Span#setStatus implementation doesn't conform to the spec.

Notably:

• Status codes form a priority of OK > ERROR > UNSET, where a call to set a lower priority shouldn't be able to override a higher priority status. Currently, its possible for a call with UNSET and override ERROR.
• Description should be ignored when status is not ERROR, but is not