qdoc: Better error messages for QML command errors

qdoc: Better error messages for QML command errors

Some error messages were not clear for these qdoc commands: \qmlclass,
\qmlmodule, \inqmlmodule, and \qmlproperty. They have been made clearer
now.

Also, qdoc now parses input files in the same order all the time now.
The order is alphabetic now. This might not be the optimal order.

Change-Id: Id53a5ec8105009c71f4bbd41973a54aed7821099
Reviewed-by: Casper van Donderen <casper.vandonderen@nokia.com>