message: signrawtransaction*: improve error for partial signing
  Thanks to Danial Jaffy (tipu) for reporting this issue.
diff: @@ -309,6 +309,9 @@ UniValue SignTransaction(CMutableTransaction& mtx, const SigningProvider* keysto
  if (serror == SCRIPT_ERR_INVALID_STACK_OPERATION) {
  // Unable to sign input and verification failed (possible attempt to partially sign).
  TxInErrorToJSON(txin, vErrors, "Unable to sign input, invalid stack size (possibly missing key)");
  + } else if (serror == SCRIPT_ERR_SIG_NULLFAIL) {
  + // Verification failed (possibly due to insufficient signatures).
  + TxInErrorToJSON(txin, vErrors, "CHECK(MULTI)SIG failing with non-zero signature (possibly need more signatures)");
  } else {
  TxInErrorToJSON(txin, vErrors, ScriptErrorString(serror));
  }
  