* Call otel.Handle with non-nil errors
That's what normally happens in other call sites. Those two didn't
check it, but passed the "error" to Handle. The default, delegating
implementation of ErrorHandler was printing the error unconditionally,
which resulted in pointless lines like `2020/12/07 19:51:28 <nil>` in
demos, for example.
* Add tests
Co-authored-by: Tyler Yahn <MrAlias@users.noreply.github.com>