unet-cli: use modern ucode syntax

unet-cli: use modern ucode syntax

Refactor various places in the script to use modern syntax, such as
template strings or `in` lookups.

Also introduce a simple `assert()` helper function to deal with the
repeated `if (!cond) { warn(msg); exit(1) }` pattern.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>