ifupdown: scheduler: ignore errors generated by upperiface bring up

ifupdown: scheduler: ignore errors generated by upperiface bring up

Ticket: CM-12923
Reviewed By: julien, nikhil
Testing Done: tested implicit upperiface bring up

upperiface bringup is best effort, so ignore errors.

$ifquery -c -a
..[snip]...
auto br0
iface br0
        bridge-vlan-aware yes                        [pass]
        bridge-ports vx-14 vx-11 vx-10 vx-13 vx-12   [pass]

$ifdown vx-10
$ifdown vx-11

before patch:
$ifup vx-10
..[snip]...
info: running upperifaces (parent interfaces) if available ..
info: br0: running ops ...
error: br0: bridge port vx-11 does not exist
....

After patch:
$ifup vx-10
..[snip]...
info: running upperifaces (parent interfaces) if available ..
info: br0: running ops ...
....

Signed-off-by: Roopa Prabhu <roopa@cumulusnetworks.com>