JCLOUDS-678: Do not silently return null in write operations if a 404 is returned

https://issues.apache.org/jira/browse/JCLOUDS-678

Write operations should not use the `NullOnNotFoundOr404`, as the resource should exist. I've kept that fallback, however, in all the DELETE operations, as it is a pattern widely used in jclouds. If the resource to be deleted does not exist, don't fail, as the *expected result* of the operation is achieved.

I've also added the missing unit and live tests for the `InstanceApi.setTags` method.

@aledsage could you kindly run the live tests for this change? I don't have an account to use for testing this with right now.