Adding on span error callbacks

[jondeandres]

I'd like to extend the functionality of Span._on_error to allow integrating other tools with the error handling of the spans.

I don't think monkey patching is the good way to go on this, since also Span._on_error is a private method.

Another approach would be having a exit_callbacks list to be executed on each __exit__ call.

I am not sure this funcionality exists on other languages implementation but it'd open the door to do more powerful integrtions with OpenTracing.