Fail with message

I think it would be useful if there was a fail method that would either compare the error that was thrown or the message. Something similar to the following:

``` java
assertThat(observable.toBlocking())
   .failsWithError(new Throwable("Some error'))
```

or

``` java
assertThat(observable.toBlocking())
   .failsWithErrorMessage("Some error")
```
