MM Auth: introduce AUTH_ERROR constant.
Instead of using hardcoded -1 for errors, include -1 in the enum auth_action
type; apply its use.
In the mm_auth test, the string output changes from '(internal error)' to
'AUTH_ERROR', since now the proper enum value is used in auth_action_names[].
diff --git a/openbsc/tests/mm_auth/mm_auth_test.ok b/openbsc/tests/mm_auth/mm_auth_test.ok
index cc0e769..7dedadc 100644
--- a/openbsc/tests/mm_auth/mm_auth_test.ok
+++ b/openbsc/tests/mm_auth/mm_auth_test.ok
@@ -1,7 +1,7 @@
* test_error()
wrapped: db_get_authinfo_for_subscr(): rc = -5
-auth_get_tuple_for_subscr(key_seq=0) --> auth_action == (internal error)
+auth_get_tuple_for_subscr(key_seq=0) --> auth_action == AUTH_ERROR
* test_auth_not_avail()
wrapped: db_get_authinfo_for_subscr(): rc = -2