Added Errorable instances to other monads.

Yes, they are orphan instances. No, I don't care.
I think the likelihood that the standard Reader/Writer/etc. monads
implement Errorable in their own land is low, and if they do then
just don't import Dangerous.Extensions.

src/Control/Dangerous/Extensions.hs

@@ -0,0 +1,30 @@
+-- This is the land of the orphans, so NO WHINING
+{-# OPTIONS_GHC -fno-warn-orphans #-}
+module Control.Dangerous.Extensions where
+import Prelude hiding ( log )
+import Control.Dangerous
+import Control.Monad.Cont
+import Control.Monad.Maybe
+import Control.Monad.Reader
+import Control.Monad.State
+import Control.Monad.Writer
+
+instance (Errorable m) => Errorable (ContT s m) where
+    log = lift . log
+    exit = lift . exit
+
+instance (Errorable m) => Errorable (MaybeT m) where
+    log = lift . log
+    exit = lift . exit
+
+instance (Monoid s, Errorable m) => Errorable (WriterT s m) where
+    log = lift . log
+    exit = lift . exit
+
+instance (Errorable m) => Errorable (ReaderT s m) where
+    log = lift . log
+    exit = lift . exit
+
+instance (Errorable m) => Errorable (StateT s m) where
+    log = lift . log
+    exit = lift . exit