ui-shared: add cgit_print_error_page() function

This will allow us to generate error responses with the correct HTTP
response code without needing all of the layout boilerplate.

Signed-off-by: John Keeping <john@keeping.me.uk>

ui-shared.c

@@ -788,6 +788,20 @@ void cgit_print_docend(void)
 	html("</body>\n</html>\n");
 }
 
+void cgit_print_error_page(int code, const char *msg, const char *fmt, ...)
+{
+	va_list ap;
+	ctx.page.status = code;
+	ctx.page.statusmsg = msg;
+	cgit_print_http_headers();
+	cgit_print_docstart();
+	cgit_print_pageheader();
+	va_start(ap, fmt);
+	cgit_vprint_error(fmt, ap);
+	va_end(ap);
+	cgit_print_docend();
+}
+
 static void add_clone_urls(void (*fn)(const char *), char *txt, char *suffix)
 {
 	struct strbuf **url_list = strbuf_split_str(txt, ' ', 0);