chore: codebase hardening

[Jon-Becker]

Use proper error handling everywhere. Heimdall should never panic, and most nontrivial functions should be made to return Result<_, _>s

Todo:

• remove unsafe .unwrap()s. (If an unwrap is safe, use .expect()
• remove all instances of std::process::exit
• implement crate::Error for each crate (heimdall-core, heimdall-cli, etc). See here for an example.
• Convert functions to Result<_, crate::Error> wherever possible