util: fix exception stringification in .format()

Fixes nodejs#1634.

lib/util.js

@@ -164,14 +164,16 @@ function inspect(obj, showHidden, depth, colors) {
       case 'boolean':
         return stylize('' + value, 'boolean');
     }
+
     // For some reason typeof null is "object", so special case here.
     if (value === null) {
       return stylize('null', 'null');
     }
 
     // Look up the keys of the object.
     var visible_keys = Object.keys(value);
-    var keys = showHidden ? Object.getOwnPropertyNames(value) : visible_keys;
+    var keys = showHidden || isException(value) ?
+        Object.getOwnPropertyNames(value) : visible_keys;
 
     // Functions without properties can be shortcutted.
     if (typeof value === 'function' && keys.length === 0) {
@@ -337,6 +339,12 @@ function isDate(d) {
 }
 
 
+function isException(e) {
+  return e instanceof Error ||
+    (typeof e === 'object' && Object.prototype.toString.call(e) === '[object Error]');
+}
+
+
 var pWarning;
 
 exports.p = function() {