ifupdownmain: run up/down on manual interfaces, but ignore any errors. (closes #48)

ifupdownmain: run up/down on manual interfaces, but ignore any errors. (closes #48)

ifupdown changed its original behavior on "manual" address method:
https://salsa.debian.org/debian/ifupdown/commit/12d333d619013914aae7deafe45fb6191ff2fb2c

ifupdown2 needs to change it's behavior too.

Reported-by: Alexandre Derumier <aderumier@odiso.com>
Reviewed-by: Roopa Prabhu <roopa@cumulusnetworks.com>
Co-authored-by: Alexandre Derumier <aderumier@odiso.com>
Co-authored-by: Julien Fortin <julien@cumulusnetworks.com>
Signed-off-by: Julien Fortin <julien@cumulusnetworks.com>