Added Linker::getInvalidTitleDescription() to display invalid title entries in variou...

Added Linker::getInvalidTitleDescription() to display invalid title entries in various places.

This method will use two new messages 'invalidtitle-knownnamespace' and 'invalidtitle-unknownnamespace' depending on the fact that the given namespace number exists or not.
I did put that method in Linker because I plan to use it in various places, notably in other QueryPage and Pager subclasses.

Change-Id: I13e7cdc2c0a8e86dc5e4b144b6012f3864d2ec06