Return 403 instead of 500 when policy check fails

This patch fixes an issue where a 500 response is sent instead of a 403
when a request is made using the wrong scope.  e.g.  Using project
scope instead of system scope.

Story: 2009170
Task: 43200

Change-Id: Id399d2220118efe1033426c658d1834cbff02f94

barbican/api/__init__.py

@@ -96,7 +96,7 @@ def generate_safe_exception_message(operation_name, excep):
 
     try:
         raise excep
-    except policy.PolicyNotAuthorized:
+    except (policy.PolicyNotAuthorized, policy.InvalidScope):
         message = u._(
             '{operation} attempt not allowed - '
             'please review your '