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/Helper/Chunked.hs

@@ -12,7 +12,7 @@ module Remote.Helper.Chunked (
 	storeChunks,
 	removeChunks,
 	retrieveChunks,
-	hasKeyChunks,
+	checkPresentChunks,
 ) where
 
 import Common.Annex
@@ -94,8 +94,8 @@ storeChunks
 	-> Key
 	-> FilePath
 	-> MeterUpdate
-	-> (Key -> ContentSource -> MeterUpdate -> Annex Bool)
-	-> (Key -> Annex (Either String Bool))
+	-> Storer
+	-> CheckPresent
 	-> Annex Bool
 storeChunks u chunkconfig k f p storer checker = 
 	case chunkconfig of
@@ -158,7 +158,7 @@ storeChunks u chunkconfig k f p storer checker =
 seekResume
 	:: Handle
 	-> ChunkKeyStream
-	-> (Key -> Annex (Either String Bool))
+	-> CheckPresent
 	-> Annex (ChunkKeyStream, BytesProcessed)
 seekResume h chunkkeys checker = do
 	sz <- liftIO (hFileSize h)
@@ -172,7 +172,7 @@ seekResume h chunkkeys checker = do
 			liftIO $ hSeek h AbsoluteSeek sz
 			return (cks, toBytesProcessed sz)
 		| otherwise = do
-			v <- checker k
+			v <- tryNonAsyncAnnex (checker k)
 			case v of
 				Right True ->
 					check pos' cks' sz
@@ -331,43 +331,48 @@ setupResume ls currsize = map dropunneeded ls
 {- Checks if a key is present in a remote. This requires any one
  - of the lists of options returned by chunkKeys to all check out
  - as being present using the checker action.
+ -
+ - Throws an exception if the remote is not accessible.
  -}
-hasKeyChunks
-	:: (Key -> Annex (Either String Bool))
+checkPresentChunks
+	:: CheckPresent
 	-> UUID
 	-> ChunkConfig
 	-> EncKey
 	-> Key
-	-> Annex (Either String Bool)
-hasKeyChunks checker u chunkconfig encryptor basek
-	| noChunks chunkconfig =
+	-> Annex Bool
+checkPresentChunks checker u chunkconfig encryptor basek
+	| noChunks chunkconfig = do
 		-- Optimisation: Try the unchunked key first, to avoid
 		-- looking in the git-annex branch for chunk counts
 		-- that are likely not there.
-		ifM ((Right True ==) <$> checker (encryptor basek))
-			( return (Right True)
-			, checklists Nothing =<< chunkKeysOnly u basek
-			)
+		v <- check basek
+		case v of
+			Right True -> return True
+			_ -> checklists Nothing =<< chunkKeysOnly u basek
 	| otherwise = checklists Nothing =<< chunkKeys u chunkconfig basek
   where
-	checklists Nothing [] = return (Right False)
-	checklists (Just deferrederror) [] = return (Left deferrederror)
+	checklists Nothing [] = return False
+	checklists (Just deferrederror) [] = error deferrederror
 	checklists d (l:ls)
 		| not (null l) = do
 			v <- checkchunks l
 			case v of
 				Left e -> checklists (Just e) ls
-				Right True -> return (Right True)
+				Right True -> return True
 				Right False -> checklists Nothing ls
 		| otherwise = checklists d ls
 	
 	checkchunks :: [Key] -> Annex (Either String Bool)
 	checkchunks [] = return (Right True)
 	checkchunks (k:ks) = do
-		v <- checker (encryptor k)
-		if v == Right True
-			then checkchunks ks
-			else return v
+		v <- check k
+		case v of
+			Right True -> checkchunks ks
+			Right False -> return $ Right False
+			Left e -> return $ Left $ show e
+
+	check = tryNonAsyncAnnex . checker . encryptor
 
 {- A key can be stored in a remote unchunked, or as a list of chunked keys.
  - This can be the case whether or not the remote is currently configured