fail_now return wrong exit code

[johscheuer]

If you use the fail_now function the return code is 0 but should be 1. The function fail does this correctly.

The problem probably is here: https://github.com/garethr/kubetest/blob/master/assert/assert.go#L185 it's called with AssertionError and all other functions are called with AssertionFailure