Inform user that partitions are not to be executed alone

https://github.com/aeronautical-informatics/apex-linux/blob/3a503936950c6e47ac293481f1ff78a87a80b89a/partition/src/lib.rs#L28

panics if one tries to execute a partition freestanding, without the hypervisor:

```console
thread '' panicked at 'called `Result::unwrap()` on an `Err` value: TypedError { err: PartitionInit, source: environment variable not found }', /sources/apex-rs-linux-0.1.0-main/src/lib.rs:28:45
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
fatal runtime error: failed to initiate panic, error 5
zsh: IOT instruction (core dumped) result/bin/taws
````
Maybe it would be clever to print a message to the user, explaining briefly what to do instead (run the partition through the hypervisor).