qapi: New documentation section tag "Errors"

qapi: New documentation section tag "Errors"

We use section "Returns" for documenting both success and error
response of commands.

I intend to generate better command success response documentation.
Easier when "Returns" documents just he success response.

Create new section tag "Errors".  The next two commits will move error
response documentation from "Returns" sections to "Errors" sections.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-ID: <20240227113921.236097-4-armbru@redhat.com>