Return Json for all Not Found views in REST API

[ephraimbuddy]

Currently if an API URL that does not exist is requested, an html page is returned instead of JSON

Apache Airflow version:2.0

What happened:
Making a request against an endpoint that does not exist returns an html page

What you expected to happen:

I expected it to return 404 error in JSON indicating that the URL was not found

How to reproduce it:

1. Start airflow website and scheduler in breeze.
2. Make a request against an endpoint that does not exist e.g http://localhost:28080/api/v1/getconnectiond
3. Notice that an html page is returned instead of a json
   cc: @mik-laj