Odra.toml error handling on empty file.

[zie1ony]

When Odra.toml is empty I get this when cargo odra test

thread 'main' panicked at /Users/ziel/.cargo/registry/src/index.crates.io-6f17d22bba15001f/cargo-odra-0.1.0/src/odra_toml.rs:60:65:
called `Result::unwrap()` on an `Err` value: Error { inner: Error { inner: TomlError { message: "missing field `contracts`", original: Some("#[[contracts]]\n#fqn = \"mycontract::Mycontract\"\n"), keys: [], span: Some(0..0) } } }
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

It should inform user that maybe the module was not registered.

[zie1ony]

Related: #37