Error handling

[XBagon]

There's a lot of error-related boilerplate which in the end doesn't do much other than just print out a single message. I'd suggest to use the crate anyhow, which would make this a lot less boilerplate, nicer to look at and faster to add new errors.

Another thing I noticed is the usage of ? in situations where an Error should absolutely impossible as seen here:

rust-music-theory/src/scale/scale.rs

Lines 40 to 44 in 7cff5d6

	let intervals = match scale_type {
	ScaleType::Diatonic => Interval::from_semitones(&[2, 2, 1, 2, 2, 2, 1]),
	ScaleType::HarmonicMinor => Interval::from_semitones(&[2, 1, 2, 2, 1, 3, 1]),
	ScaleType::MelodicMinor => Interval::from_semitones(&[2, 1, 2, 2, 2, 2, 1]),
	}?;

I think just unwraping here would be the better choice to signal the impossibility of failing and to be 100% sure to be noticed when it fails, cause then probably something went terrible wrong.