Handle manifests with missing tools section more gracefully

[filiptibell]

This PR closes #12 by properly checking if the [tools] section exists before further reading of Rokit manifests.

[kennethloeffler]

LGTM - are there possibly other places where TOML documents are indexed directly?

[filiptibell]

are there possibly other places where TOML documents are indexed directly?

Not to my knowledge - Aftman manifests use serde, Foreman's manifest and Rokit's auth manifest both look fine. Those should be all of the other manifests we deserialize / index into. I did a quick grep to make sure as well.