Wallet throws on missing signature by default

Using dummy sigs by default may be confusing for multi-sig scenarios.
See: https://groups.google.com/forum/#!topic/bitcoinj/PEhZGk3WMxU
Signers: Share keypaths for watching wallets

Now LocalTransactionSigner populates ProposedTransaction.keyPaths map even if
there is no private key in the wallet. Other signers may take advantage
of that.