I had to do this recently as well – what I did was to use (in Nuxeo Studio) the “User Interface -> Add Message” instruction in the automation chain, checking the response of my custom operation in the expression language to set the severity/message text. E.g.

message:

Packaging request @{Context.input != "" ? "succeeded" : "failed"}. 

severity:

@{Context.input != "" ? "INFO" : "ERROR"}

My operation returns a nonempty string on success, but the above worked for simple booleans too with the correct expression.

If you're not using Nuxeo Studio there's surely a way to do this involving editing XML.