[AIRFLOW-4341] Remove `View.render()` already exists in fab.BaseView (#5125)
[AIRFLOW-4341] Remove `View.render()` already exists in fab.BaseView (#5125)

This behaviour is exactly what the fab.BaseView.render_template function
already does, and since that exists it makes more sense to use that than
define our own function

The two error handlers have been moved out of the class since even
though they _looked_ like they were instance methods they weren't: the
`self` argument was actually an error object.
danielvdende/incubator-airflow,apache/incubator-airflow,dhuang/incubator-airflow,bolkedebruin/airflow,r39132/airflow,mrkm4ntr/incubator-airflow,Fokko/incubator-airflow,airbnb/airflow,wooga/airflow,DinoCow/airflow,dhuang/incubator-airflow,danielvdende/incubator-airflow,bolkedebruin/airflow,Acehaidrey/incubator-airflow,cfei18/incubator-airflow,apache/airflow,cfei18/incubator-airflow,danielvdende/incubator-airflow,spektom/incubator-airflow,airbnb/airflow,wileeam/airflow,nathanielvarona/airflow,danielvdende/incubator-airflow,airbnb/airflow,nathanielvarona/airflow,lyft/incubator-airflow,wooga/airflow,mtagle/airflow,mistercrunch/airflow,Fokko/incubator-airflow,bolkedebruin/airflow,lyft/incubator-airflow,mistercrunch/airflow,nathanielvarona/airflow,dhuang/incubator-airflow,spektom/incubator-airflow,spektom/incubator-airflow,apache/incubator-airflow,mtagle/airflow,Fokko/incubator-airflow,sekikn/incubator-airflow,r39132/airflow,DinoCow/airflow,DinoCow/airflow,wooga/airflow,apache/incubator-airflow,nathanielvarona/airflow,Acehaidrey/incubator-airflow,cfei18/incubator-airflow,mrkm4ntr/incubator-airflow,wileeam/airflow,cfei18/incubator-airflow,mrkm4ntr/incubator-airflow,cfei18/incubator-airflow,DinoCow/airflow,Acehaidrey/incubator-airflow,Acehaidrey/incubator-airflow,mtagle/airflow,wileeam/airflow,lyft/incubator-airflow,r39132/airflow,r39132/airflow,sekikn/incubator-airflow,wooga/airflow,mistercrunch/airflow,danielvdende/incubator-airflow,mistercrunch/airflow,mrkm4ntr/incubator-airflow,apache/airflow,dhuang/incubator-airflow,mtagle/airflow,Fokko/incubator-airflow,apache/airflow,bolkedebruin/airflow,apache/incubator-airflow,danielvdende/incubator-airflow,airbnb/airflow,apache/airflow,lyft/incubator-airflow,cfei18/incubator-airflow,apache/airflow,nathanielvarona/airflow,bolkedebruin/airflow,Acehaidrey/incubator-airflow,wileeam/airflow,spektom/incubator-airflow,Acehaidrey/incubator-airflow,sekikn/incubator-airflow,apache/airflow,nathanielvarona/airflow,sekikn/incubator-airflow