Remove Alpha3Error

[reez]

Describe the enhancement

Just creating a tracking Issue for removing the Alpha3Error, which will mean that we have implemented specific error types for everything needed in bdk-ffi

Additional context
I currently have a few open draft PR's on this so thought it was a good idea to have a tracking Issue

TODO

• format
• alphabetize?
• re-order UDL & error.rs
• tests
• update UDL with proper section headings for some of these (for example, under keys mnemonic from_string/entropy & pathconstructor should not be under descriptor module in UDL)
• naming of error strings (e/err/error/error_message)
• For the “from”s should we type alias to bdkXYZError or should we be fully explicit like bdk::bitcoin::psbt::ExtractTxError?
• For this error should we do use thiserror::Error;