[enchrichment] support span eventName `error`

[neptunian]

Should we support error event name in addition to exception?

Semconv states the eventName must be exception however there seems to be a lot of usage of events with error eventName not being picked up. I experience this with the otel-demo, described in this issue open-telemetry/opentelemetry-demo#2905.

A search also reveals some usages: https://github.com/search?q=span.AddEvent%28%22Error%22%2C+trace.WithAttributes%28+language%3AGo&type=code