Side effect of set_prolog_flag(iso, true)

Error messages don’t show correctly anymore. With set_prolog_flag(iso, false)
it gives me error message in top-level:

?- set_prolog_flag(iso, false).

?- X is msb(0).
ERROR: Domain error: `not_less_than_one' expected, found `0'
ERROR:   [10] _120 is msb(0)
ERROR:    [9] toplevel_call(user:user: ...)
at d:/software/logic/swipl/boot/

With set_prolog_flag(iso, true) it has problems showing the error message:

?- set_prolog_flag(iso, true).

?- X is msb(0).
ERROR: '$clause'/4: No permission to access private_procedure `toplevel_call/1'

Thanks. Pushed a fix for that. That said, a lot of stuff is falling down in ISO mode as large parts of the tooling is incompatible with the ISO mode.