Add last_response attr to WaiterError

[jamesls]

The rationale behind just adding a last_response instead of exposing some sort
of error_code and error_message is that a WaiterError can also be thrown in
non-error states (transitioning to a failure state, hitting max-attempts), and
while we can always guarantee a last response, we can't necessarily guarantee
that response is a "error" response.

Closes #957

[codecov-io]

Current coverage is 97.56% (diff: 100%)

Merging #1023 into develop will increase coverage by <.01%

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[kyleknap]

Looks good. I like having the last response for any potential waiter error as opposed to just errors propogated from service errors. 🚢

[JordonPhillips]

This will make debugging waiter issues much easier

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