Error for invalid targets when editing spamlist

This shows a simple error message for invalid targets, instead of
silently ignoring them as before. I considered using a Status object to
show details (which targets are invalid, invalid title or wiki), but
that isn't practical until we move away from FormSpecialPage.

Followup to Ibddb6e2cf547e5d158eba45df0c3ed0c6f56bf42

Change-Id: I3c29391c4fe3f21c92c61f61fc545a260b43a9b8