The verse discusses Nevi'im who prophesize evil, but not teach anything about them!
Malbim: Most Nevi'im prophesized evil, so Yisrael would repent. There is no need to prophesize about good, if not to establish a Navi to be a Navi Emes 1 . The Torah (Devarim 18:21-22) says that if a Navi's words are not fulfilled, Hashem did not say so. This refers to a prophecy for good. If a prophecy for bad is not fulfilled, perhaps the decree was changed because people repented.
Elisha was already established to be a Navi Emes when he prophesized at a time of starvation that tomorrow, the price of flour in Shomron will plummet (Melachim II, 7:1)! Perhaps this was to assuage the king, who wanted to kill Elisha. (PF)
