Capture span's (and not transaction's) context for errors on span

[SergeyKleyman]

Derived from https://github.com/elastic/apm-dev/issues/390.

Currently .NET agent captures only transaction's context for a error even if the error in captured on a span which has its own context - span's context is just ignored. All this is according to current intake API spec.

Java agent team considers to include span's context in the captured error as well in some way - elastic/apm-agent-java#279. We might want to do that as well.

[stevejgordon]

Closing as no plans to implement this currently.