Discuss and sign off on error API

[tcharding]

Fix error APIs

ref: #43 (comment)

@Kixunil is not around at the moment and I do not know what is meant by this comment of his.

We want to 1.0 this crate, we should discuss and sign off on the error API.

Discussion should include:

• C-GOOD-ERR
• How public we want the errors

IIRC, fully closing all the errors in this crate makes us have to duplicate the error types in rust-bitcoin. I did this at some stage in the past and it was a massive PITA. IMO we should just make a decision here and accept that we are stuck with it for 1.0