make retrieveKeyFile and retrieveKeyFileCheap throw exceptions

Converted retrieveKeyFileCheap to a Maybe, to avoid needing to throw a
exception when a remote doesn't support it.

Command/Get.hs

@@ -112,5 +112,9 @@ getKey' key afile = dispatch
 		download (Remote.uuid r) key afile stdRetry
 			(\p -> do
 				showAction $ "from " ++ Remote.name r
-				Remote.retrieveKeyFile r key afile dest p
+				tryNonAsync (Remote.retrieveKeyFile r key afile dest p) >>= \case
+					Right v -> return (True, v)
+					Left e -> do
+						warning (show e)
+						return (False, UnVerified)
 			) witness