API calls: raise original exception when retries exhausted

This is an alternative approach to solve the problem identified
in #279 . When doing API calls with `retry_on_error` set to
true (the default), if the call fails after all retries are
exhausted, we raise our own `MaximumRetriesExceeded` exception,
which tells you nothing about what actually went wrong, just
that we exhausted the retry count. It seems much more useful
to raise an exception with information about why the connection
is failing.

This changes that behaviour so that, on most paths, we would
raise an appropriate exception from requests (either the one
we caught, or an HTTPError via `raise_for_status`) if we exhaust
the retry count. Only if we fail due to database lag would we
still raise `MaximumRetriesExceeded` (because there isn't really
an underlying exception we can handily raise in that case).

Note that it was already possible to hit
`stream.raise_for_status()` before this change, if the status
code was outside the 500 range, so in theory callers should
already be prepared for that. It was not possible to get a
`requests.exceptions.ConnectionError` or
`requests.exceptions.Timeout` with `retry_on_error` true, though,
so this is genuinely a behaviour change on that front.

Signed-off-by: Adam Williamson <awilliam@redhat.com>