kernel: Add fatalError method to notifications

FatalError replaces what previously was the AbortNode function in
shutdown.cpp.

This commit is part of the libbitcoinkernel project and further removes
the shutdown's and, more generally, the kernel library's dependency on
interface_ui with a kernel notification method. By removing interface_ui
from the kernel library, its dependency on boost is reduced to just
boost::multi_index. At the same time it also takes a step towards
de-globalising the interrupt infrastructure.

Co-authored-by: Russell Yanofsky <russ@yanofsky.org>
Co-authored-by: TheCharlatan <seb.kung@gmail.com>

src/node/kernel_notifications.h

@@ -7,6 +7,7 @@
 
 #include <kernel/notifications_interface.h>
 
+#include <atomic>
 #include <cstdint>
 #include <string>
 
@@ -18,6 +19,8 @@ namespace node {
 class KernelNotifications : public kernel::Notifications
 {
 public:
+    KernelNotifications(std::atomic<int>& exit_status) : m_exit_status{exit_status} {}
+
     void blockTip(SynchronizationState state, CBlockIndex& index) override;
 
     void headerTip(SynchronizationState state, int64_t height, int64_t timestamp, bool presync) override;
@@ -27,6 +30,13 @@ public:
     void warning(const bilingual_str& warning) override;
 
     void flushError(const std::string& debug_message) override;
+
+    void fatalError(const std::string& debug_message, const bilingual_str& user_message = {}) override;
+
+    //! Useful for tests, can be set to false to avoid shutdown on fatal error.
+    bool m_shutdown_on_fatal_error{true};
+private:
+    std::atomic<int>& m_exit_status;
 };
 } // namespace node