http-api: T6326: return full warning/error output through api

Configuration error output is not returned in full to the http-api when
running under vyos-configd, due to an early implementation 'workaround'
of vyos-configd writing directly to the session tty. This is corrected
to return all ambient stdout (notably vyos.base.Warning) and error
messages directly to the originating caller, which may be from a session
tty or a ConfigSession instance. As the http-api runs in the latter
case, the full output is returned.