Cumulus-message-adapter

Fixed
- Increase number of configured retries in case of sfn ClientError.[CUMULUS-920]

Fixed
- Fixed rare case where `"exception": null` obscured failures.

Changed
- Exceptions that occur are now stored in the remote message.

Fixed
- Exceptions stored in the local message (e.g. by the client libraries) are carried through when the remote message is loaded.

Fixed
- Fix invocation as subprocess.

Fixed
- Improve backwards compatibility and add tests.