`os.IsNotExist()` is from before `errors.Is()` existed. The latter is the
recommended approach, as it also recognised wrapped errors.
No functional changes, except for recognising more cases of "does not
exist" errors as such.
If there is a `scripts` directory next to the current executable, load
scripts from that directory as well.
It is still required to restart the Manager in order to pick up changes
to those scripts (including new/removed files), PLUS a refresh in the
add-on.
