Add IfMatchMissing exception

Needed to make testing easier.

rest_framework/exceptions.py

@@ -96,6 +96,14 @@ class PreconditionFailed(APIException):
         self.detail = detail or self.default_detail
 
 
+class IfMatchMissing(APIException):
+    default_detail = 'IF_MATCH header is required'
+    status_code = 400
+
+    def __init__(self, detail=None):
+        self.detail = detail or self.default_detail
+
+
 class ConfigurationError(Exception):
     """
     Indicates an internal server error.

rest_framework/views.py

@@ -401,7 +401,7 @@ class APIView(View):
             if getattr(self, 'use_etags', False) and request.method.lower() in ('put', 'delete'):
                 self.etag_header = request.META.get('HTTP_IF_MATCH')
                 if self.etag_header is None:
-                    return Response({'detail': 'IF_MATCH header is required'}, status=400, exception=True)
+                    raise exceptions.IfMatchMissing
 
             response = handler(request, *args, **kwargs)