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"
Above this line, it seems like the question mark check should occur before we parse the function property shorthand.

Also, I just realized the error message is "A object member..." instead of "An object member..."