better error message when git config fails to parse remote config

Rather than leaking the name of the temp file, just say the config parse
failed, and where the config was downloaded from.

Not closing the bug report because two issues were reported in the same
bug report, because the universe wants me to continually re-read old
unclosed bug reports to waste my time determining what still needs to be
done.

Git/Config.hs

@@ -1,6 +1,6 @@
 {- git repository configuration handling
  -
- - Copyright 2010-2019 Joey Hess <id@joeyh.name>
+ - Copyright 2010-2020 Joey Hess <id@joeyh.name>
  -
  - Licensed under the GNU AGPL version 3 or higher.
  -}
@@ -14,6 +14,7 @@ import qualified Data.ByteString as S
 import qualified Data.ByteString.Char8 as S8
 import Data.Char
 import qualified System.FilePath.ByteString as P
+import Control.Concurrent.Async
 
 import Common
 import Git
@@ -184,19 +185,22 @@ coreBare = "core.bare"
 
 {- Runs a command to get the configuration of a repo,
  - and returns a repo populated with the configuration, as well as the raw
- - output of the command. -}
-fromPipe :: Repo -> String -> [CommandParam] -> IO (Either SomeException (Repo, S.ByteString))
+ - output and any standard output of the command. -}
+fromPipe :: Repo -> String -> [CommandParam] -> IO (Either SomeException (Repo, S.ByteString, S.ByteString))
 fromPipe r cmd params = try $
-	withHandle StdoutHandle createProcessSuccess p $ \h -> do
-		val <- S.hGetContents h
+	withOEHandles createProcessSuccess p $ \(hout, herr) -> do
+		geterr <- async $ S.hGetContents herr
+		getval <- async $ S.hGetContents hout
+		val <- wait getval
+		err <- wait geterr
 		r' <- store val r
-		return (r', val)
+		return (r', val, err)
   where
 	p = proc cmd $ toCommand params
 
 {- Reads git config from a specified file and returns the repo populated
  - with the configuration. -}
-fromFile :: Repo -> FilePath -> IO (Either SomeException (Repo, S.ByteString))
+fromFile :: Repo -> FilePath -> IO (Either SomeException (Repo, S.ByteString, S.ByteString))
 fromFile r f = fromPipe r "git"
 	[ Param "config"
 	, Param "--file"