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/Git.hs

@@ -141,8 +141,8 @@ gen r u c gc
 			, retrieveKeyFile = copyFromRemote new
 			, retrieveKeyFileCheap = copyFromRemoteCheap new
 			, removeKey = dropKey new
-			, hasKey = inAnnex new
-			, hasKeyCheap = repoCheap r
+			, checkPresent = inAnnex new
+			, checkPresentCheap = repoCheap r
 			, whereisKey = Nothing
 			, remoteFsck = if Git.repoIsUrl r
 				then Nothing
@@ -284,11 +284,8 @@ tryGitConfigRead r
 			void $ tryAnnex $ ensureInitialized
 			Annex.getState Annex.repo
 
-{- Checks if a given remote has the content for a key inAnnex.
- - If the remote cannot be accessed, or if it cannot determine
- - whether it has the content, returns a Left error message.
- -}
-inAnnex :: Remote -> Key -> Annex (Either String Bool)
+{- Checks if a given remote has the content for a key in its annex. -}
+inAnnex :: Remote -> Key -> Annex Bool
 inAnnex rmt key
 	| Git.repoIsHttp r = checkhttp
 	| Git.repoIsUrl r = checkremote
@@ -298,17 +295,13 @@ inAnnex rmt key
 	checkhttp = do
 		showChecking r
 		ifM (Url.withUrlOptions $ \uo -> anyM (\u -> Url.checkBoth u (keySize key) uo) (keyUrls rmt key))
-			( return $ Right True
-			, return $ Left "not found"
+			( return True
+			, error "not found"
 			)
 	checkremote = Ssh.inAnnex r key
-	checklocal = guardUsable r (cantCheck r) $ dispatch <$> check
-	  where
-		check = either (Left . show) Right 
-			<$> tryAnnex (onLocal rmt $ Annex.Content.inAnnexSafe key)
-		dispatch (Left e) = Left e
-		dispatch (Right (Just b)) = Right b
-		dispatch (Right Nothing) = cantCheck r
+	checklocal = guardUsable r (cantCheck r) $
+		fromMaybe (cantCheck r)
+			<$> onLocal rmt (Annex.Content.inAnnexSafe key)
 
 keyUrls :: Remote -> Key -> [String]
 keyUrls r key = map tourl locs'