Check that key on S3 exists before attempting deploy

[alulsh]

If you run lambda-cfn update <stack> or lambda-cfn create <stack> and the key for the Lambda code on S3 has not been uploaded yet (either manually or as part of an automated build + deploy system) then lambda-cfn will error during the CloudFormation deploy process (and not cancel the deploy). This means a user will have gone through the CloudFormation parameter input process and deploy confirmation process only to have a failed deploy at the final step during the CloudFormation deloy. This process takes a while and can be frustrating.

To make this module a little more user friendly we could have lambda-cfn first check that the code exists on S3 after providing parameter values (in order to obtain the location on S3) but before the final CloudFormation deploy.

/cc @ianshward @zmully