Specify the default value for the SEND_ERROR_EMAIL setting. Closes #341.

docs/configuration.rst

@@ -1072,6 +1072,8 @@ The default value for the `Task.send_error_emails` attribute, which if
 set to :const:`True` means errors occurring during task execution will be
 sent to :setting:`ADMINS` by email.
 
+Disabled by default.
+
 .. setting:: ADMINS
 
 ADMINS