add go templating engine for error page and make errors more clear (#260)

ping #199
closes #213

Reviewed-on: https://codeberg.org/Codeberg/pages-server/pulls/260
Co-authored-by: crapStone <crapstone01@gmail.com>
Co-committed-by: crapStone <crapstone01@gmail.com>

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, "", ctx.StatusCode)
+		html.ReturnErrorPage(ctx, "gitea client failed", ctx.StatusCode)
 	}
 }