Clarify error message on policy presubmit

Reword the error message shown when a Policy lacks a required field.

Change-Id: I654ee95f2b5e2a57f9bd31f2626900240a1ef121
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2529696Reviewed-by: Chris Sharp <csharp@chromium.org>
Reviewed-by: Julian Pastarmov <pastarmovj@chromium.org>
Commit-Queue: Eric Lawrence [MSFT] <ericlaw@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#826442}

components/policy/tools/syntax_check_policy_template_json.py

@@ -1480,7 +1480,8 @@ class PolicyTemplateChecker(object):
       # policies instead of just new ones.
       if self._NeedsDefault(new_policy) and new_policy.get('default',
                                                            None) == None:
-        self._Error(('Missing default field for policy %s') % (new_policy_name))
+        self._Error("Definition of policy %s must include a 'default'"
+                    " field." % (new_policy_name))
 
   def _LeadingWhitespace(self, line):
     match = LEADING_WHITESPACE.match(line)