logging: Fix log filter during initialization

The log filter was being ignored on initialization due to the logging instance being initialized before the config instance, so the log filter was set to its default value.

This fixes that oversight, along with using descriptive exceptions instead of abort() calls.

src/core/core.cpp

@@ -4,6 +4,7 @@
 
 #include <array>
 #include <atomic>
+#include <exception>
 #include <memory>
 #include <utility>
 
@@ -423,9 +424,16 @@ struct System::Impl {
 System::System() : impl{std::make_unique<Impl>(*this)} {}
 System::~System() = default;
 
+System& System::GetInstance() {
+    if (!s_instance) {
+        throw std::runtime_error("Using System instance before its initialization");
+    }
+    return *s_instance;
+}
+
 void System::InitializeGlobalInstance() {
     if (s_instance) {
-        abort();
+        throw std::runtime_error("Reinitializing Global System instance.");
     }
     s_instance = std::unique_ptr<System>(new System);
 }