Do not ever fail hard when configuring interfaces

[Maximilian Wilhelm]

Currently we fail hard and exit an executor is something fails (thanks to set -e). This can easily lead to situations where neither ìfup nor ifdown will move forward or backward and you have to manually intervene. This is bad and it shouldn't be that way with ifupdown-ng.

So I propose to remove set -e from any executors so we can carry on when adding/removing an address/route fails for whatever reason (already present/removed?) and to prevent dead locks.

Eventually if would be nice to have "intelligent executors" which only touch parts which have to be changed to make configuration reality.