audit: improve error message on schema failures

If the audit schema fails, it is most probably caused by an audit of a
newer Bob version. Add a hint to the user that an update might help.

pym/bob/audit.py

@@ -338,7 +338,8 @@ def load(self, file, name):
                 r["artifact-id"] : Artifact.fromData(r) for r in tree["references"]
             }
         except schema.SchemaError as e:
-            raise ParseError(name + ": Invalid audit record: " + str(e))
+            raise ParseError(name + ": Invalid audit record: " + str(e),
+                             help="Try updating to the latest Bob version. The audit probably contains new record types.")
         except ValueError as e:
             raise ParseError(name + ": Invalid json: " + str(e))
         self.__validate()