fix(test): ensure that grunt test:client exits with non-zero code

Apparently it is not enough to check `code`, because when process
crashes `code` is set to `null` and `err` is passed instead.

Example of failing tests, which didn't fail build:
https://travis-ci.org/karma-runner/karma/jobs/537027667#L1046

tasks/test.js

@@ -26,7 +26,7 @@ module.exports = function (grunt) {
 
     function exec (args, failMsg) {
       spawnKarma(args, function (err, result, code) {
-        if (code) {
+        if (code || err) {
           console.error(err)
           grunt.fail.fatal(failMsg, code)
         } else {