Numerous changes to get api methods into a more uniform/consistant shape.
Numerous changes to get api methods into a more uniform/consistant shape.

1. Handle extraction of message items in a stylized and consistant way.
2. Do not prepare to call engine method until message extraction is complete.
3. Always handle result of engine method with two branches.

* Use a convenience function to get return code and message.

This general format should be followed for the remaing methods.

Signed-off-by: mulhern <7b51bcf507bcd7afb72bf8663752c0ddbeb517f6@redhat.com>

mulkieran/stratisd,trgill/stratisd,stratis-storage/stratisd,stratis-storage/stratisd,stratis-storage/stratisd,trgill/stratisd,mulkieran/stratisd