bdk::Wallet should have context-specific error types

[evanlinjin]

          I agree that the errors aren't great.

However, I disagree about having a single error enum with a large number of variants. I think we should fix errors in one go with a separate PR, otherwise it's duplicate work.

Originally posted by @evanlinjin in #976 (comment)

[LLFourn]

I think the bdk::Error enum should go but I don't know when that can happen. I'd probably put it last on the list of things needing to be done. All new fallible functions should have their own error type.

[notmandatory]

Moving this to milestone alpha.4 where we're going to focus on Wallet APIs.

[nondiremanuel]

As per what we said during the last Lib Team Call, the fix for this should be into #1028 PR