Explain why sign button is disabled

[Sjors]

The sign button is disabled when psbt.canSign returns false. We should make it more clear why this happens. This may require a PR to libwally-swift.

canSign is implemented here:
https://github.com/blockchain/libwally-swift/blob/master/LibWally/PSBT.swift#L54-L73

Some reasons why it can fail:

• wrong network: the network is not specified in the PSBT, but you can tell from the derivation path, e.g. m/84h/0h is mainnet and m/84h/1h is testnet
• change address derivation is not what we expect
• we don't have matching keys (wrong derivation path?); this might be where we should check if any of the other wallets do match

[w-i-n-s]

some details #53 (comment)

[w-i-n-s]

@Sjors Yes, we need additional commits which adds more information to the returned value. Instead of just returning nil as an error-cased result. Some swift language details here.

Personally, I would recommend use Result<[PubKey: KeyOrigin], PBSTSignError>
where enum PBSTSignError: Error {}