Add a per node error handler possibility

examples/15_node_error_handler/main.cpp

@@ -0,0 +1,73 @@
+// Example 15 — Per-node Error Handler
+//
+// Demonstrates set_error_handler() for deciding whether a network can
+// continue when a node throws an exception.
+//
+// The "validator" node rejects even numbers by throwing std::runtime_error.
+// Its error handler logs the failure and returns true (skip & continue),
+// so odd numbers still flow through to the sink.
+//
+// Compare: a second handler (commented below) returns false instead,
+// which stops the node and gracefully shuts the downstream side down.
+//
+// Pipeline:  [source] --int--> [validator] --int--> [sink]
+
+#include <kpn/kpn.hpp>
+#include <chrono>
+#include <iostream>
+#include <thread>
+
+static int counter = 0;
+
+static int source() {
+    std::this_thread::sleep_for(std::chrono::milliseconds(20));
+    return ++counter;
+}
+
+static int validate(int x) {
+    if (x % 2 == 0)
+        throw std::runtime_error("even number rejected: " + std::to_string(x));
+    return x;
+}
+
+static int received = 0;
+
+static void sink(int x) {
+    std::cout << "  processed: " << x << '\n';
+    ++received;
+}
+
+int main() {
+    using namespace kpn;
+
+    auto src  = make_node<source>  ();
+    auto proc = make_node<validate>();
+    auto snk  = make_node<sink>   ();
+
+    // Return true  → skip this invocation, keep the node running.
+    // Return false → stop the node (downstream drains then also stops).
+    proc.set_error_handler([](std::string_view name, std::exception_ptr ep) {
+        try { std::rethrow_exception(ep); }
+        catch (const std::exception& e) {
+            std::cerr << "[" << name << "] skipping item — " << e.what() << '\n';
+        }
+        return true;
+    });
+
+    Network net;
+    net.add("source",    src)
+       .add("validator", proc)
+       .add("sink",      snk)
+       .connect("source",    src.output<0>(),  "validator", proc.input<0>())
+       .connect("validator", proc.output<0>(), "sink",      snk.input<0>())
+       .build();
+
+    std::cout << "source emits 1..N; validator rejects even numbers.\n"
+              << "Error messages on stderr, accepted items on stdout.\n\n";
+
+    net.start();
+    std::this_thread::sleep_for(std::chrono::milliseconds(300));
+    net.stop();
+
+    std::cout << "\nItems accepted by sink: " << received << '\n';
+}