refactor: have a structured IndexOutOfBoundsError type

[chaitika]

Description

Fixes #267

Notes to the reviewers

I can rebase to master after #298 is merged to fix fmt errors.

Changelog notice

Checklists

All Submissions:

• I've signed all my commits
• I followed the contribution guidelines
• I ran just p before pushing

New Features:

• I've added tests for the new feature
• I've added docs for the new feature

Bugfixes:

• This pull request breaks the existing API
• I've added tests to reproduce the issue which are now passing
• I'm linking the issue being fixed by this PR

[coveralls]

Pull Request Test Coverage Report for Build 17246072754

Details

• 27 of 43 (62.79%) changed or added relevant lines in 4 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.07%) to 84.843%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
wallet/src/descriptor/policy.rs	8	11	72.73%
wallet/src/types.rs	3	8	37.5%
wallet/src/wallet/signer.rs	13	21	61.9%

Totals
Change from base Build 17132535147:	-0.07%
Covered Lines:	6689
Relevant Lines:	7884

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💛 - Coveralls

[ValuedMammal]

Concept ACK

I really like that you were able to find places where the new type could be substituted in. If this change were to only add the IndexOutOfBoundsError then I think we could accept it straight away, but since it also touches the public API by changing the PolicyError and SignerError enums, then it may need to be pushed to 3.0. That said, it may not be worth adding features to policy.rs since development has shifted to the miniscript::plan module.

[110CodingP]

Ack 1e86fc1!
Ran the tests and checked that the correct error message is produced (by removing the should_panic attribute in the psbt tests.)