Sanity check $wgNamespaceAliases

If someone goes and adds aliases for namespaces that don't actually
exist (as was done in I94c34799, for example), Scribunto will run into
issues when trying to create its mw.site.namespace objects.

Let's ignore those bogus aliases so we don't go breaking everything just
because someone did something stupid.

Change-Id: I16acd97f587de320cf61becb829cc66794cbb119