Clean-up: coding style fix and missing method in lttng::ctl::error

Methods must not contain get/set for trivial accessors. get_code() is
renamed to code() to follow this guideline.

Moreover, the method was not implemented. It is marked as noexcept and
the _error_code member is made `const` as there is no reason for it to
be mutated.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: If0fca5cc91c1cb72fff183ee6d29588ff4b5c030