Document message response codes in request datatypes

[adrianghc]

Currently, message response codes are not documented in the request datatypes in the include/hems/messages headers, but instead as return values for the handler functions for these request messages. This is a very flawed approach from an API documentation standpoint, as a developer wanting to send a message to a module currently needs to look in two places in the code: One to learn the format of the message and a separate place to learn the possible responses. Instead, possible message response codes need to be documented (additionally or exclusively) in the request datatype declarations.