Clarify 'before execution begins' in response

Author: benjie

spec/Section 7 -- Response.md

@@ -47,6 +47,10 @@ not be present in the result.
 If an error was raised during the execution that prevented a valid
 response, the `data` entry in the response should be `null`.
 
+Note: the {ExecuteRequest()} algorithm allows ["request
+errors"](#sec-Errors.Request-errors) to be raised "before execution begins,"
+these should result in the `data` entry not being present in the result.
+
 
 ### Errors