Fix false positive mailmotor subscribe/unsubscribe result

When the subscribe or unsubscribe function fails and returns false, the
error should be triggered. Otherwise the end-user gets a false positive
success message.

This should fix ticket #336

And added another fix for a problem that bugged me for some time. When ampersands or quotes are used in page titles, the internal link list was pretty much unreadable. This was easily fixed by decoding html entities from that list.