Improve error message, if model can't be loaded

[hannobraun]

Here's the old error message, as of before #1234

thread 'main' panicked at 'Error reloading model: LibLoading(DlOpen { desc: "/home/hanno/Projects/main/fornjot/target/debug/libspacer.so: cannot open shared object file: No such file or directory" })', /home/hanno/.cargo/registry/src/github.com-1ecc6299db9ec823/fj-host-0.19.0/src/lib.rs:322:25
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

And this is the new one, as of this pull request:

Error receiving updated shape: Error loading model from dynamic library

Caused by:
    /home/hanno/Projects/main/fornjot/target/debug/libspacer.so: cannot open shared object file: No such file or directory