Call function `subject` and expect the function to raise an exception.
    >>> expect = Expector([])
    >>> expect(lambda: 1 / 0).to(raise_error, ZeroDivisionError)
    (True, 'Expect ZeroDivisionError to be raised')

def raise_error(subject, error_class):
    '''
    Call function `subject` and expect the function to raise an exception.
    >>> expect = Expector([])
    >>> expect(lambda: 1 / 0).to(raise_error, ZeroDivisionError)
    (True, 'Expect ZeroDivisionError to be raised')
    '''
    description = 'Expect {} to be raised'.format(error_class.__name__)
    try:
        subject()
        return (False, description)
    except error_class:
        return (True, description)