Adding sanity check to Title::isRedirect().

Adding sanity check to Title::isRedirect().

isRedirect() assumes that the LinkCache already contains
information about this title. If that is not the case, it
currently returns false, even though it just doesn't know
whether this link is a redirect.

The new check asserts the assumption that this title
is already known to the link cache.

Amend: use Exception instead of assert()

Change-Id: Id3ad2d4e140b270b1f5ca1f7af9b3320cffff5a2