Handle errors from external scripts

[kpcyrd]

The python example in the readme currently doesn't do any error handling, so a network failure would not result in a retry but marks the attempt as failed.

The python example should use a different error code for exceptions and the lua wrapper should handle this error.

• there should be a way to test if there has been an error by querying the runtime state
• if the lua script returns an error while an error has already been recorded, the error should be chained. Error { last_error, next: Error { previous_error } }
• error handling should be unified, if the lua function returns a string, this should cause an Err(_)