Improve error handling in the XML/XSL processor

[schst]

If a main_*.xml file contains invalid XML, the processor will throw strange errors:

Catchable fatal error: Argument 1 passed to stubXSLProcessor::setXMLDocument() must be an instance of DOMDocument, boolean given, called in C:\wamp\www\projects\stubbles\src\main\php\net\stubbles\xml\stubXMLXIncludeStreamWrapper.php on line 172 and defined in C:\wamp\www\projects\stubbles\src\main\php\net\stubbles\xml\xsl\stubXSLProcessor.php on line 76

The same applies to missing images.

The error handling should be improved, as mostly producers will receive those messages.

[schst]

XML/XSL component will be part of 0.3.0

[mikey]

Fixed with changeset 842.