Bug#911727: initramfs-tools-core: configure_networking avoid panic and wait for at least one network interface.

[Greek <greek64.mail@gmail.com>]

This bug is over 2 years old, and still not handled.

The 'configure_networking' function should not panic if the network 
cannot be configured, but fail gracefully.
It should be the responsibility of the caller to handle the fail and 
re-try if appropriate.

I increased the severity of the bug.