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.)

Command/AddUrl.hs

@@ -197,8 +197,7 @@ startWeb o urlstring = go $ fromMaybe bad $ parseURI urlstring
 		pathmax <- liftIO $ fileNameLengthLimit "."
 		urlinfo <- if relaxedOption (downloadOptions o)
 			then pure Url.assumeUrlExists
-			else Url.withUrlOptions $
-				liftIO . Url.getUrlInfo urlstring
+			else Url.withUrlOptions $ Url.getUrlInfo urlstring
 		file <- adjustFile o <$> case fileOption (downloadOptions o) of
 			Just f -> pure f
 			Nothing -> case Url.urlSuggestedFile urlinfo of