Well what about Jonah then? The things he prophesied didn't come to pass because the Ninevites repented...
So was Jonah wrong? Was he lying? Was he proven a false prophet, lying about the word God had given him? Of course not. God makes it clear in the law that repentance of sin will turn aside his wrath. Had the people of Nineveh went along their way, they would have been destroyed.