[bdk_chain_redesign] List chain data methods can be try/non-try

[bdk_chain_redesign] List chain data methods can be try/non-try

Methods that list chain data have try and non-try versions. Both of
these versions now return an `Iterator`.

* Try versions return `Iterator<Item = Result>`.
* Non-try versions require the `ChainOracle` implementation to be
  `ChainOracle<Error = Infallible>`.