#4 Recoverable loading operations

[Jean-Jacques Puig]

syntax/*Parser.php classes implement generateModel() kind of methods, some of which open a stream or die if they fail to do so. This may trigger unrecoverable errors (scripts ending prematurely) in the application using RAP.
This patch updates parsers exposing this behavior in order to allow continuous execution. Furthermore, the Model class and subclasses implementing load() methods are updated so that their return value (previously void, now boolean) reflects the success or failure of the load() operation.

[Jean-Jacques Puig]