MINOR: trace: split the CLI "trace" parser in CLI vs statement

MINOR: trace: split the CLI "trace" parser in CLI vs statement

In order to be able to reuse the "trace" statements elsewhere (e.g.
global section), we'll first need to split its parser. It turns out
that the whole thing is self-contained inside a single function that
emits a single message on warning/error or nothing on success. That's
quite easy to split in two parts, the one that does the job and produces
the status message and the one that sends it to the CLI. That's what
this patch does.