Merge pull request #347 from MangoIV/mangoiv/fix-monadfail-instance

[fix] add proper MonadFail instance

Author: echatav

squeal-postgresql/src/Squeal/PostgreSQL/Session.hs

@@ -214,9 +214,9 @@ instance (Monad m, db0 ~ db1)
   return = pure
   (>>=) = flip pqBind
 
-instance (Monad m, db0 ~ db1)
+instance (MonadFail m, db0 ~ db1)
   => Fail.MonadFail (PQ db0 db1 m) where
-  fail = Fail.fail
+  fail = lift . Fail.fail
 
 instance db0 ~ db1 => MFunctor (PQ db0 db1) where
   hoist f (PQ pq) = PQ (f . pq)