utils: fix process races when killing processes (#5721)
utils: fix process races when killing processes (#5721)

airflow.utils.helpers.reap_process_group() can throw uncaught OSErrors
if processes exit at the wrong time in its execution. Fix this by
catching all OSErrors that can arise due to a process race, and
returning from them when the error is ESRCH (process not found).
Fokko/incubator-airflow,lyft/incubator-airflow,Acehaidrey/incubator-airflow,apache/incubator-airflow,mistercrunch/airflow,nathanielvarona/airflow,cfei18/incubator-airflow,bolkedebruin/airflow,cfei18/incubator-airflow,mrkm4ntr/incubator-airflow,DinoCow/airflow,DinoCow/airflow,cfei18/incubator-airflow,wileeam/airflow,Acehaidrey/incubator-airflow,danielvdende/incubator-airflow,dhuang/incubator-airflow,danielvdende/incubator-airflow,apache/incubator-airflow,Acehaidrey/incubator-airflow,DinoCow/airflow,mistercrunch/airflow,bolkedebruin/airflow,lyft/incubator-airflow,mtagle/airflow,mrkm4ntr/incubator-airflow,sekikn/incubator-airflow,DinoCow/airflow,danielvdende/incubator-airflow,mrkm4ntr/incubator-airflow,spektom/incubator-airflow,apache/incubator-airflow,airbnb/airflow,mrkm4ntr/incubator-airflow,dhuang/incubator-airflow,sekikn/incubator-airflow,nathanielvarona/airflow,Fokko/incubator-airflow,dhuang/incubator-airflow,apache/incubator-airflow,mtagle/airflow,danielvdende/incubator-airflow,mtagle/airflow,Fokko/incubator-airflow,sekikn/incubator-airflow,sekikn/incubator-airflow,Acehaidrey/incubator-airflow,Acehaidrey/incubator-airflow,Fokko/incubator-airflow,cfei18/incubator-airflow,lyft/incubator-airflow,bolkedebruin/airflow,lyft/incubator-airflow,wooga/airflow,wooga/airflow,dhuang/incubator-airflow,wileeam/airflow,apache/airflow,cfei18/incubator-airflow,airbnb/airflow,cfei18/incubator-airflow,spektom/incubator-airflow,apache/airflow,nathanielvarona/airflow,apache/airflow,wooga/airflow,bolkedebruin/airflow,airbnb/airflow,wooga/airflow,apache/airflow,mistercrunch/airflow,apache/airflow,nathanielvarona/airflow,mistercrunch/airflow,wileeam/airflow,apache/airflow,danielvdende/incubator-airflow,danielvdende/incubator-airflow,spektom/incubator-airflow,nathanielvarona/airflow,bolkedebruin/airflow,Acehaidrey/incubator-airflow,spektom/incubator-airflow,nathanielvarona/airflow,mtagle/airflow,wileeam/airflow,airbnb/airflow