PSBT dialog: adding non-final signature to input results in error

[Sjors]

Only tried this with a legacy wallet.

Steps:

1. Create 3 legacy wallets
2. Call getnewaddress and getaddressinfo, note the public key (a, b, c)
3. On each of the wallets, call addmultisigaddress 2 [a,b,c] "Multisig" legacy
4. Copy the inferred descriptor returned by any of the previous calls, drop the origin info
5. Create a watch-only descriptor wallet and import the descriptor
6. Send coins to it, wait for confirmation
7. Create a PSBT in the GUI by sending the funds elsewhere (without a change address)
8. Open this PSBT in wallet a, b or c and try to sign it
9. Notice the error message: "Could not sign any more inputs"
10. Try instead to sign via RPC with walletprocesspsbt using one of the wallets
11. Copy the processed result and open it in one of the other two wallets
12. Now it happily signs it, because the 2nd signature makes it complete

cc @achow101