SecretsManager: Add some error messages for BatchGetSecretValue

[connor9]

For the batch_get_secret_value call, adds error reporting for secrets that aren't found or those that are marked as deleted. These will be included in the format returned from the AWS call:

{
  'SecretValues': [],
  'Errors': [
    {
      'SecretId': 'secret-id-arn',
      'ErrorCode': 'ResourceNotFoundException',
      'Message': "Secrets Manager can't find the specified secret."
    }
  ],
  'ResponseMetadata': ...
}

[cloudlena]

I'd love to see this! What's the current state, @connor9? Can I somehow support?