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"
This seems slightly worse; if we see a semi-colon in these contexts, we really should be assuming they meant a comma and gracefully continue (and assume they meant whatever we were able to parse - that means this would be a shorthand property).