fix (network.fetchError): define errorText as implementation defined string

Author: juliandescottes

index.bs

@@ -9052,7 +9052,8 @@ error</dfn> steps given |request|:
   1. Let |params| be the result of [=process a network event=] with |session|
      "<code>network.fetchError</code>", and |request|.
 
-  1. TODO: Set the <code>errorText</code> field of |params|.
+  1. Set the <code>errorText</code> field of |params| to an implementation-defined string
+     describing the error which caused the request to be aborted.
 
   1. Assert: |params| matches the <code>network.FetchErrorParameters</code>
      production.