Improve error messages in parsing for short-hand property assignment

Instead of report "Cannot find name ..." in short-hand property assignment because we try to parse it as short-hand property assignment, it is more desirable to assume users try to write normal property assignment and report ": expected"
The similar argument can be made for normal property assignment too. The main reason I change this is that trying to parse this as normal short-hand so we don't report missing identifier as well.