pushed checkPresent exception handling out of Remote implementations

I tend to prefer moving toward explicit exception handling, not away from
it, but in this case, I think there are good reasons to let checkPresent
throw exceptions:

1. They can all be caught in one place (Remote.hasKey), and we know
   every possible exception is caught there now, which we didn't before.
2. It simplified the code of the Remotes. I think it makes sense for
   Remotes to be able to be implemented without needing to worry about
   catching exceptions inside them. (Mostly.)
3. Types.StoreRetrieve.Preparer can only work on things that return a
   Bool, which all the other relevant remote methods already did.
   I do not see a good way to generalize that type; my previous attempts
   failed miserably.

Remote/Glacier.hs

@@ -52,8 +52,8 @@ gen r u c gc = new <$> remoteCost gc veryExpensiveRemoteCost
 			retrieveKeyFile = retreiveKeyFileDummy,
 			retrieveKeyFileCheap = retrieveCheap this,
 			removeKey = remove this,
-			hasKey = checkPresent this,
-			hasKeyCheap = False,
+			checkPresent = checkKey this,
+			checkPresentCheap = False,
 			whereisKey = Nothing,
 			remoteFsck = Nothing,
 			repairRepo = Nothing,
@@ -164,25 +164,21 @@ remove r k = glacierAction r
 	, Param $ archive r k
 	]
 
-checkPresent :: Remote -> Key -> Annex (Either String Bool)
-checkPresent r k = do
+checkKey :: Remote -> Key -> Annex Bool
+checkKey r k = do
 	showAction $ "checking " ++ name r
 	go =<< glacierEnv (config r) (uuid r)
   where
-	go Nothing = return $ Left "cannot check glacier"
+	go Nothing = error "cannot check glacier"
 	go (Just e) = do
 		{- glacier checkpresent outputs the archive name to stdout if
 		 - it's present. -}
-		v <- liftIO $ catchMsgIO $ 
-			readProcessEnv "glacier" (toCommand params) (Just e)
-		case v of
-			Right s -> do
-				let probablypresent = key2file k `elem` lines s
-				if probablypresent
-					then ifM (Annex.getFlag "trustglacier")
-						( return $ Right True, untrusted )
-					else return $ Right False
-			Left err -> return $ Left err
+		s <- liftIO $ readProcessEnv "glacier" (toCommand params) (Just e)
+		let probablypresent = key2file k `elem` lines s
+		if probablypresent
+			then ifM (Annex.getFlag "trustglacier")
+				( return True, error untrusted )
+			else return False
 
 	params = glacierParams (config r)
 		[ Param "archive"
@@ -192,7 +188,7 @@ checkPresent r k = do
 		, Param $ archive r k
 		]
 
-	untrusted = return $ Left $ unlines
+	untrusted = unlines
 			[ "Glacier's inventory says it has a copy."
 			, "However, the inventory could be out of date, if it was recently removed."
 			, "(Use --trust-glacier if you're sure it's still in Glacier.)"