zcash_client_backend: Faster failure detection for invalid sent transactions

[daira]

I believe that if we currently send a transaction through lightwalletd, and it is invalid (for example, a double spend) but the backing full node only detects the invalidity asynchronously, then zcash_client_backend-based wallets will have to wait until the transaction expires before reporting it as having failed. This should be able to be reported much more quickly.