Allow hiding stack trace

Fixes error when stack trace is hidden with ```Error.stackTraceLimit = 0```
Potentially provides a fix to caolan#64

Author: iahs

lib/utils.js

@@ -205,6 +205,8 @@ exports.betterErrors = function (assertion) {
         );
         var spacing = (multiline ? '\n' : ' ');
         e._message = e.message;
+        // avoids error when stack trace is hidden with Error.stackTraceLimit
+        e.stack = e.stack || '';
         e.stack = (
             e.name + ':' + spacing +
             actual + spacing + e.operator + spacing +