Fail gracefully if settings can't be persisted

[fhunleth]

When trying to configure networking and the data partition isn't working, the following exception is raised:

** (File.Error) could not make directory (with -p) "/root/vintage_net": read-only file system
    (elixir 1.13.4) lib/file.ex:316: File.mkdir_p!/1
    (vintage_net 0.12.1) lib/vintage_net/persistence/flat_file.ex:24: VintageNet.Persistence.FlatFile.save/2
    (vintage_net 0.12.1) lib/vintage_net/interface.ex:112: VintageNet.Interface.configure/3

This is a really bad error, but it seems harsh to not let the network be configured since that might be the way to get a firmware update to fix the issue.