common: don't abort() if wally_psbt_output_taproot_keypath_add() fails.

It fails on duplicates.  It would ideally succeed, but bug reported:

	https://github.com/ElementsProject/libwally-core/issues/509

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Changelog-Fixed: JSON-RPC: `signpsbt` no longer crashes if asked to sign an already-signed PSBT with taproot paths.

tests/test_wallet.py

@@ -948,7 +948,6 @@ def test_sign_external_psbt(node_factory, bitcoind, chainparams):
     l1.rpc.signpsbt(psbt)
 
 
-@pytest.mark.xfail(strict=True)
 def test_sign_signed_psbt(node_factory, bitcoind, chainparams):
     l1 = node_factory.get_node()
     l1.fundwallet(10**6)