More user-friendly error message when partially signing

src/rpc/rawtransaction.cpp

@@ -873,7 +873,12 @@ UniValue signrawtransaction(const JSONRPCRequest& request)
 
         ScriptError serror = SCRIPT_ERR_OK;
         if (!VerifyScript(txin.scriptSig, prevPubKey, &txin.scriptWitness, STANDARD_SCRIPT_VERIFY_FLAGS, TransactionSignatureChecker(&txConst, i, amount), &serror)) {
-            TxInErrorToJSON(txin, vErrors, ScriptErrorString(serror));
+            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 {
+                TxInErrorToJSON(txin, vErrors, ScriptErrorString(serror));
+            }
         }
     }
     bool fComplete = vErrors.empty();