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/bitcoin-chainstate.cpp

@@ -101,7 +101,11 @@ int main(int argc, char* argv[])
         {
             std::cerr << "Error flushing block data to disk: " << debug_message << std::endl;
         }
-
+        void fatalError(const std::string& debug_message, const bilingual_str& user_message) override
+        {
+            std::cerr << "Error: " << debug_message << std::endl;
+            std::cerr << (user_message.empty() ? "A fatal internal error occurred." : user_message.original) << std::endl;
+        }
     };
     auto notifications = std::make_unique<KernelNotifications>();