Fix support for "unsupported" p2sh schemes. This codebase claims to not make any particular effort to support parsing of standard multisig scripts, but we still don't want the application to crash or show an error message when users paste an externally generated valid hex transaction.
This was likeky broken in the recent major `Transaction` refactoring. The problem is that `TxInput.type` can be undefined (`None`) in some cases, and the `TxInput.to_coin_dict` function expects it to be always defined.
Cleanup the TxInput.to_coin_dict function slightly: `parse_scriptsig` function no longer raises (at least intentionnaly, if it does it is a bug that needs to be fixed), so those few lines of code were unreachable. Now that we set `txin.type = ScriptType.unknown` in parse_scriptsig, we can use this to detect the case of unsupported p2sh.