Fix the agent's error collector not being reset after forking

In the `after_fork` hook, stats are supposed to be reset to be prevent the child process from reporting data collected by the parent.  While the `unsent_errors` get reset, the error collector does not -- which means if the parent process is reporting errors, there's a possibility that the children can report the same errors.  This updates `reset_stats` to also reset the errors that have been collected.