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/Web.hs

@@ -50,8 +50,8 @@ gen r _ c gc =
 		retrieveKeyFile = downloadKey,
 		retrieveKeyFileCheap = downloadKeyCheap,
 		removeKey = dropKey,
-		hasKey = checkKey,
-		hasKeyCheap = False,
+		checkPresent = checkKey,
+		checkPresentCheap = False,
 		whereisKey = Just getUrls,
 		remoteFsck = Nothing,
 		repairRepo = Nothing,
@@ -98,12 +98,12 @@ dropKey k = do
 	mapM_ (setUrlMissing k) =<< getUrls k
 	return True
 
-checkKey :: Key -> Annex (Either String Bool)
+checkKey :: Key -> Annex Bool
 checkKey key = do
 	us <- getUrls key
 	if null us
-		then return $ Right False
-		else return =<< checkKey' key us
+		then return False
+		else either error return =<< checkKey' key us
 checkKey' :: Key -> [URLString] -> Annex (Either String Bool)
 checkKey' key us = firsthit us (Right False) $ \u -> do
 	let (u', downloader) = getDownloader u