Extract exception handler usage to separate method.

This allow using custom exception handler on view class level,
and thus having differently configured APIs in one project.

Solves multi-configuration problem for django-rest-framework-json-api
that has a custom exception handler that must return jsonapi compatible
error response.

rest_framework/views.py

@@ -425,10 +425,8 @@ class APIView(View):
             else:
                 exc.status_code = status.HTTP_403_FORBIDDEN
 
-        exception_handler = self.settings.EXCEPTION_HANDLER
-
         context = self.get_exception_handler_context()
-        response = exception_handler(exc, context)
+        response = self._get_error_response(exc, context)
 
         if response is None:
             raise
@@ -436,6 +434,9 @@ class APIView(View):
         response.exception = True
         return response
 
+    def _get_error_response(self, exc, context):
+        return self.settings.EXCEPTION_HANDLER(exc, context)
+
     # Note: Views are made CSRF exempt from within `as_view` as to prevent
     # accidental removal of this exemption in cases where `dispatch` needs to
     # be overridden.