add a __str__ implementation to APIException

Add a __str__ implementation to rest_framework.exceptions.APIException. This helps for logging raised exceptions. Thanks.

rest_framework/exceptions.py

@@ -20,6 +20,8 @@ class APIException(Exception):
     def __init__(self, detail=None):
         self.detail = detail or self.default_detail
 
+    def __str__(self):
+        return self.detail
 
 class ParseError(APIException):
     status_code = status.HTTP_400_BAD_REQUEST