ensure hides actual exception coverage

[gurukamath]

I have been analysing if we can identify the need for writing additional tests (or expanding the current tests that we support) based not the coverage report from codecov.

I've found that the ensure statements in the specs hide the real coverage of a check that we perform.

For example - line 511 in the frontier/spec.py file
The codecov report shows that the line is covered. However, the single line ensure hides whether the InvalidBlock exception is actually ever triggered here. Turns out that it never is triggered.

There are a few other instances as above.

I propose that we replace all ensure statements in the spec with multiline checks of the format. That way we can identify exactly which checks have no real "hits"

if check:
      raise InvalidBlock

@SamWilsn @petertdavies What do you think?