arch: arm: fix error reason intialization

This commit fixes a minor bug by setting the error reason
(to, default, _NANO_ERR_HW_EXCEPTION) for errors detected
during Secure/Non-Secure stack inspection. Applicable only
to Secure firmware.

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>