Add explanation for this exception mutation

rest_framework/fields.py

@@ -74,6 +74,9 @@ def get_attribute(instance, attrs):
             try:
                 instance = instance()
             except (AttributeError, KeyError) as exc:
+                # If we raised an Attribute or KeyError here it'd get treated
+                # as an omitted field in `Field.get_attribute()`. Instead we
+                # raise a ValueError to ensure the exception is not masked.
                 raise ValueError('Exception raised in callable attribute "{0}"; original exception was: {1}'.format(attr, exc))
 
     return instance