Emit good error message, if model doesn't define model function

[hannobraun]

As of now, when loading a model that doesn't define a model function, this is what happens:

thread 'main' panicked at 'Error reloading model: LibLoading(DlSym { desc: "target/debug/liberror.so: undefined symbol: model" })', 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 good error message that explains what's wrong and how to fix it. The way to fix this, is to define a model function in the model's lib.rs:

#[fj::model]
pub fn model() -> fj::Shape {
    // ...
}