Remove unused exception in SpecialPage::getTitleFor and add tests

The way the existing code was, it seems the exception condition
was never hit, as one could enter a bogus special page title in
SpecialPage::getTitleFor and it returns a Title object anyway.

Some tests are also added for SpecialPage::getTitleFor(),
although more refactoring my be required to truly unit test this,
vs. indirectly testing other stuff via static methods.

Change-Id: I6e427d047a2f8e58677eb65b50866f767078c255
Merge "Remove unused exception in SpecialPage::getTitleFor and add tests"