Gracefully handle validate when an unknown pack is referenced

[perryqh]

Before

thread 'main' panicked at src/packs/pack_set.rs:124:41:
/Users/perry.hertler/workspace/zenpayroll/packs/partners_api/package.yml has 'packs/tacos' in its dependencies, but that pack cannot be found. Try `packs list-packs` to debug.
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

After

1 validation error(s) detected:
/Users/perry.hertler/workspace/zenpayroll/packs/partners_api/package.yml has 'packs/tacos' in its dependencies, but that pack cannot be found. Try `packs list-packs` to debug.

[perryqh]

If we're OK with bringing anyhow in, I'll follow this PR up with one that replaces the existing std::result usages

[alexevanczuk]

Looks good thank you! Also you'll want to bump the version when you're ready for a release, feel free to send another PR :)

Thanks for your continued work.

The new dependency looks interesting. Seems like it is a better wrapper around the error features that ship with Rust vs other solutions which attempt to rebuild error handling... looks fine to me! Would be curious to hear what you like about it in your own words :)

[perryqh]

anyhow:

I really like how it infers the returned error and easily propagates errors with an optional "context". I've also noticed that some of the big names in the rust community use anyhow by default.