SignPSBTInput will need to report the specific things that caused an
error to callers, so change it to return a PSBTError. Additionally some
callers will now check the return value and report an error to the user.
Currently, this should not change any behavior as the things that
SignPBSTInput will error on are all first checked by its callers.
Add separate PSBTError enum instead of reusing TransactionError enum for PSBT
operations, and drop unused error codes. The error codes returned by PSBT
operations and transaction broadcast functions mostly do not overlap, so using
an unified enum makes it harder to call any of these functions and know which
errors actually need to be handled.
Define PSBTError in the common library because PSBT functionality is
implemented in the common library and used by both the node (for rawtransaction
RPCs) and the wallet.
