worker: actually track error metrics on worker handle error

Created by: Strum355

Actually track workers' handle error in the error total metric src_<banana>_processor_error_total. Previously it wasnt doing that, because we were returning nil from handle if no error or only a handle error occurred. How did we not ever catch this lolmao

Test plan

I tested this manually with these conditions:

1. handle errors dont bubble up beyond the call to .handle()
2. infra errors do bubble up
3. handle errors are counted in src__processor_error_total
4. infra errors are also counted in the above