Fixes for error propagation

[hudlow]

Fixes #113.

@slott56 I'm going to need some help with this — it causes the flow logs test to fail, but I think there's something wrong with the test. Unfortunately, I just don't understand the c7n features well enough to debug it.

[hudlow]

Need to investigate the lint failure...

[slott56]

src/celpy/celtypes.py:446: error: Redundant cast to "Iterable[int]"  [redundant-cast]

I'm unclear why this isn't passing. It looks like a new lint version is being used and it doesn't like this redundant cast.

My suggestion is to fix the failing line:

            return super().__new__(cls, cast(Iterable[int], source))

Remove the cast.

[slott56]

These changes look good.

There is an essentially unrelated change required by a lint failure.

See the comments on the PR for the change that needs to be folded into this change.