fix(reporter): prevent throwing exception when null is sent to formatter

In particular tests, the createErrorFormatter was recieving `null` instead of a string, causing the reporter to throw an exception and crash karma entirely. This was only happening in Firefox 22 - and I'm pretty sure it's due to the assert library I'm using, but better to protect against it.
Merge pull request #633 from remy/fix-firefox-22-mac-null-replace-exception

fix(reporter): prevent throwing exception when null is sent to formatter