Have a feditest.report_problem() or such

Common pattern: something is supposed to raise an exception, but does not and continues normally:

```
try:
    this_should_raise_exception()

    feditest.report_problem('Exception not raised')
except ExpectedException as ex:
    ...
```

Hamcrest has a construct for this, but it is non-obvious how to use it. This is simpler, but does not prevent anybody from using the Hamcrest construct either.