Emit good error message, if model package is not a dynamic library

[hannobraun]

As of now, this is what happens when you try to load a model (via fj-app -m path/to/model) that doesn't build a dynamic library:

thread 'main' panicked at 'Error reloading model: LibLoading(DlOpen { desc: "target/debug/liberror.so: cannot open shared object file: No such file or directory" })', crates/fj-host/src/lib.rs:279:25
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

This should result in a nice error message that explains what it wrong and how to fix it. The fix would be to add the following to the model's Cargo.toml:

[lib]
crate-type = ["cdylib"]

[hannobraun]

As an alternative to emitting a good error message, it might be possible to circumvent this problem completely, and not require the crate-type in the model's Cargo.toml. This could be done by using cargo rustc to compile models, and pass the --crate-type argument. This argument was just stabilized: rust-lang/cargo#10838

[hannobraun]

Closing in favor of #938.