url size fixes

addurl: Improve message when adding url with wrong size to existing file.
Before the message suggested the url didn't exist.

Fixed handling of URL keys that have no recorded size. Before, if the key
has no size, the url also had to not declare any size, which was unlikely
and wrong, or it was taken to not exist. This probably would mostly affect
keys that were added to the annex with addurl --relaxed.

Remote/Git.hs

@@ -241,7 +241,7 @@ inAnnex r key
   where
 	checkhttp headers = do
 		showChecking r
-		ifM (anyM (\u -> Url.withUserAgent $ Url.check u headers (keySize key)) (keyUrls r key))
+		ifM (anyM (\u -> Url.withUserAgent $ Url.checkBoth u headers (keySize key)) (keyUrls r key))
 			( return $ Right True
 			, return $ Left "not found"
 			)