Opentracing.io API allows marking of span being an error. We could use this in order to mark certain spans as errors. The easiest way to do this is to capture any exceptions happening during remote method call we instrument and mark as error in this case.
For this to work we must update our instrumentation to pass caught exception as result of method invocation (currently we pass null). Or we could add exception flag to the hooks, signaling that exception was thrown, but this requires more work.