applier: add missing diag_set on region_alloc failure

In case if we're hitting memory limit allocating triggers
we should setup diag error to prevent nil dereference
in diag_raise call (for example from applier_apply_tx).

Note that there are region_alloc_xc helpers which are
throwing errors but as far as I understand we need the
rollback action to process first instead of immediate
throw/catch thus we use diag_set.

Acked-by: Sergey Ostanevich <sergos@tarantool.org>
Acked-by: Konstantin Osipov <kostja.osipov@gmail.com>
Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>