Expose results rather than panicking

[mcandre]

As a developer, I want more functions to return Result instead of panicking, so that I can react to errors more gracefully.

Expose more result values downstream rather than always panicking. This has the benefit of allowing more complex workflows. Would also present a more direct error message when results are unwrapped, instead of making it look like tinyrick is erroring.

At the extreme end of API openness, how to do this in a way that doesn't require so much .unwrap() boilerplate in the user's tinyrick.rs? Perhaps we could offer both panicking / implicitly unwrapping functions, as well as result-returning functions, similar to the exec! vs. exec_mut! macros.