catch ClientError from withClientM

When getting from a P2P HTTP remote, prompt for credentials when required,
instead of failing.

This feels like it might be a bug in servant-client. withClientM's type
suggests it would not throw a ClientError. But it does in this case.

P2P/Http/Client.hs

@@ -9,6 +9,7 @@
 
 {-# LANGUAGE BangPatterns #-}
 {-# LANGUAGE DataKinds, TypeApplications #-}
+{-# LANGUAGE ScopedTypeVariables #-}
 {-# LANGUAGE CPP #-}
 
 module P2P.Http.Client (
@@ -99,7 +100,7 @@ p2pHttpClientVersions allowedversion rmt fallback clientaction =
 	versions = filter allowedversion allProtocolVersions
 	go clientenv mcred credcached mauth (v:vs) = do
 		myuuid <- getUUID
-		res <- clientaction clientenv v
+		res <- catchclienterror $ clientaction clientenv v
 			(B64UUID (uuid rmt))
 			(B64UUID myuuid)
 			[]
@@ -126,6 +127,8 @@ p2pHttpClientVersions allowedversion rmt fallback clientaction =
 					("git-annex HTTP API server returned an unexpected response: " ++ show clienterror)
 	go _ _ _ _ [] = return Nothing
 
+	catchclienterror a = a `catch` \(ex :: ClientError) -> pure (Left ex)
+
 	authrequired clientenv vs = do
 		cred <- prompt $ 
 			inRepo $ Git.getUrlCredential credentialbaseurl