fix some error messages

Author: cloudhead

lib/vows.js

@@ -77,7 +77,7 @@ function addVow(vow) {
         if (vow.callback.length >= 2 || !batch.suite.options.error) {
             runTest([err]);
         } else {
-            output('errored', { type: 'promise', error: err });
+            output('errored', { type: 'promise', error: err.stack || err.message || JSON.stringify(err) });
         }
         vows.tryEnd(batch);
     });
@@ -98,7 +98,7 @@ function addVow(vow) {
             if (e.name && e.name.match(/AssertionError/)) {
                 output('broken', e.toString());
             } else {
-                output('errored', e.stack || e.message || e.toString() || e);
+                output('errored', e.stack || e.message || e);
             }
         }
     }