make --json-error-messages capture url download errors

Convert Utility.Url to return Either String so the error message can be
displated in the annex monad and so captured.

(When curl is used, its errors are still not caught.)

Remote/Git.hs

@@ -352,11 +352,10 @@ inAnnex' repo rmt (State connpool duc _ _) key
 	checkhttp = do
 		showChecking repo
 		gc <- Annex.getGitConfig
-		ifM (Url.withUrlOptions $ \uo -> liftIO $
-			anyM (\u -> Url.checkBoth u (keySize key) uo) (keyUrls gc repo rmt key))
-				( return True
-				, giveup "not found"
-				)
+		ifM (Url.withUrlOptions $ \uo -> anyM (\u -> Url.checkBoth u (keySize key) uo) (keyUrls gc repo rmt key))
+			( return True
+			, giveup "not found"
+			)
 	checkremote = 
 		let fallback = Ssh.inAnnex repo key
 		in P2PHelper.checkpresent (Ssh.runProto rmt connpool (cantCheck rmt) fallback) key