Refactor: make descriptors::key::DescriptorKeyParseError an enum

[chris-ricketts]

This PR has a go at refactoring DescriptorKeyParseError into an enum, as suggested in the FIXME comment:

rust-miniscript/src/descriptor/key.rs

Lines 327 to 330 in d0da327

	/// Descriptor Key parsing errors
	// FIXME: replace with error enums
	#[derive(Debug, PartialEq, Clone, Copy)]
	pub struct DescriptorKeyParseError(&'static str);

I have separated the error variants by different inner parse error types and result types (e.g, public key, private key, etc) where I think it made sense to retain the inner error.

I left a variant that wraps a &'static str and its used for the various ad-hoc parsing errors instead of adding the boilerplate for variants only created once and without retained data.

The generic parse_xkey_deriv function now takes a parse function as its first parameter to allow specific key parsing errors:

https://github.com/chris-ricketts/rust-miniscript/blob/790b08fcf46f53ce6ee80355a7db8e5108ed860f/src/descriptor/key.rs#L935-L952

[apoelstra]

instead of adding the boilerplate for variants only created once and without retained data.

Can you add the boilerplate? I know it's annoying to write but it makes things much more maintainable since it forces a visible API change whenever we add or change variants.

[chris-ricketts]

Can you add the boilerplate? I know it's annoying to write but it makes things much more maintainable since it forces a visible API change whenever we add or change variants.

@apoelstra no problem, please see 895da06

Before the PR I had them all as 'top-level' error variants and it was a bit unwieldily, I think the above is a better middle ground.

[apoelstra]

In 895da06:

You need to re-export the new error type MalformedKeyDataKind from src/descriptor/mod.rs. Unfortunately I think the compiler will then immediately whine that you need a doccomment on every single variant. I'd be ok with just sticking #[allow(missing_docs)] on the enum since it's a C-like enum where every variant is named what it is.

[apoelstra]

In 895da06 and 790b08f:

Can you run through all your error messages and change them to start with a lowercase letter?

[apoelstra]

Other than those two comments 895da06 looks great! Thanks for grinding through this.

[chris-ricketts]

Thanks for the review @apoelstra

• Re-export MalformedKeyDataKind: bf058c9

• Allow missing docs for MalformedKeyDataKind: a5c1785

• Start all error messages will lower case letter: 0e98dd2

Let me know if you would prefer all the commits in this PR to be squashed into one.

[apoelstra]

Yes please -- can you just squash everything?

[chris-ricketts]

Sorted and noted for the future

[apoelstra]

ACK 497c5a3; successfully ran local tests; thanks!