Fix masked error message from Gitea

This would yield to the error "forge client failed" instead of e.g. "404 Not Found". The issue was introduced in cbb2ce6d07.

server/handler/try.go

@@ -41,7 +41,7 @@ func tryUpstream(ctx *context.Context, giteaClient *gitea.Client,
 
 	// Try to request the file from the Gitea API
 	if !options.Upstream(ctx, giteaClient, redirectsCache) {
-		html.ReturnErrorPage(ctx, "forge client failed", ctx.StatusCode)
+		html.ReturnErrorPage(ctx, fmt.Sprintf("Forge returned %d %s", ctx.StatusCode, http.StatusText(ctx.StatusCode)), ctx.StatusCode)
 	}
 }