KeychainTxOutIndex method should behave consistently when non-existent keychains are passed in as arguments

[danielabrozzoni]

Some KeychainTxOutIndex methods panic if a non-existent keychain is passed in as an argument. Some examples include:

• unbounded_spk_iter
• next_index

Instead, the methods that return an iterator simply return an empty one. Some examples are:

• keychain_outpoints
• unused_keychain_spks

I think we should be consistent, and either panic everywhere, or return a Result/Option everywhere. I personally don't like returning empty iterators, as the mistake of passing a non-existent keychain might be overlooked.

More discussion here: #1203 (comment)

[collins-okafor]

Can I work on this? Although I will need some briefing

[LLFourn]

starting work on this will need to wait until #1203 is merged.