Add arg validation to :pageinfo.

common/content/buffer.js

@@ -1423,7 +1423,13 @@ var Buffer = Module("buffer", {
 
         commands.add(["pa[geinfo]"],
             "Show various page information",
-            function (args) { buffer.showPageInfo(true, args[0]); },
+            function (args) {
+                let arg = args[0];
+                let opt = options.get("pageinfo");
+
+                dactyl.assert(opt.validator(opt.parse(arg)), "E475: Invalid argument: " + arg);
+                buffer.showPageInfo(true, arg);
+            },
             {
                 argCount: "?",
                 completer: function (context) {