Error when file is empty or commented out

### Expected behavior

Skipp empty or commented out file
### Actual behavior

printed out msg in this instance: { originalErr: {} }

```
Fails with error
Missing error handler on `socket`.
TypeError: (msg || "").replace is not a function
at C:\Work\Pixie\src\Pixie\node_modules\karma\lib\reporter.js:46:23
at onBrowserError (C:\Work\Pixie\src\Pixie\node_modules\karma\lib\reporters\base.js:58:60)
at null. (C:\Work\Pixie\src\Pixie\node_modules\karma\lib\events.js:13:22)
at emitTwo (events.js:100:13)
at emit (events.js:185:7)
at onKarmaError (C:\Work\Pixie\src\Pixie\node_modules\karma\lib\browser.js:95:13)
at Socket. (C:\Work\Pixie\src\Pixie\node_modules\karma\lib\events.js:13:22)
at emitOne (events.js:95:20)
at Socket.emit (events.js:182:7)
at Socket.onevent (C:\Work\Pixie\src\Pixie\node_modules\socket.io\lib\socket.js:335:8)
at Socket.onpacket (C:\Work\Pixie\src\Pixie\node_modules\socket.io\lib\socket.js:295:12)
at Client.ondecoded (C:\Work\Pixie\src\Pixie\node_modules\socket.io\lib\client.js:193:14)
at Decoder.Emitter.emit (C:\Work\Pixie\src\Pixie\node_modules\component-emitter\index.js:134:20)
at Decoder.add (C:\Work\Pixie\src\Pixie\node_modules\socket.io-parser\index.js:247:12)
at Client.ondata (C:\Work\Pixie\src\Pixie\node_modules\socket.io\lib\client.js:175:18)
at emitOne (events.js:90:13)
```
### Enviroment Details
- Karma version (output of `0.13.22`):
- Relevant part of your `karma.config.js` file

```
files: [
// paths loaded by Karma
{ pattern: "node_modules/angular2/bundles/angular2-polyfills.js", included: true, watched: true },
{ pattern: "node_modules/systemjs/dist/system.src.js", included: true, watched: true },
{ pattern: "node_modules/rxjs/bundles/Rx.js", included: true, watched: true },
{ pattern: "node_modules/angular2/bundles/angular2.dev.js", included: true, watched: true },
{ pattern: "node_modules/angular2/bundles/testing.dev.js", included: true, watched: true },
{ pattern: "node_modules/angular2/bundles/http.dev.js", included: true, watched: true },
{ pattern: "node_modules/angular2/bundles/router.dev.js", included: true, watched: true },
{ pattern: "karma-test-shim.js", included: true, watched: true },

// paths loaded via module imports
{ pattern: "wwwroot/app/**/*.js", included: false, watched: true },

// paths to support debugging with source maps in dev tools
{ pattern: "app/**/*.ts", included: false, watched: false },
{ pattern: "wwwroot/app/**/*.js.map", included: false, watched: false }
],
// test results reporter to use
// possible values: 'dots', 'progress'
// available reporters: https://npmjs.org/browse/keyword/karma-reporter
reporters: ["progress", "coverage"],

// default value
coverageReporter:
{
includeAllSources: true,
type: "json",
dir: "coverage",
subdir: "json",
file: "coverage-final.json"
},
```
### Steps to reproduce the behaviour
1. Have working tests
2. Comment out or remove content of entire test file
3. run karma