MostRecentProvider._get_most_recent_version does not return the correct value if the lock cannot be acquired

[mattsb42-aws]

_get_most_recent_version used to return a tuple of the version and the provider. We changed that at some point to only return a provider, but we apparently missed updating the branch taken if the lock cannot be acquired.

https://github.com/awslabs/aws-dynamodb-encryption-python/blob/master/src/dynamodb_encryption_sdk/material_providers/most_recent.py#L245-L250