Create a class for deferred error logging (#390)

## Link to GitHub Issue, if one exists
#388 

## Description of change
Instead of setting flags in `signal` and logging when we crash, create a
class for logging deferred semi-fatal messages, and dump them to the log
immediately before crashing or shutting down.

## Testing
Tested each failure case by injection.

src/spice2x/launcher/shutdown.cpp

@@ -7,6 +7,7 @@
 #include "easrv/easrv.h"
 #include "rawinput/rawinput.h"
 #include "hooks/audio/audio.h"
+#include "util/deferlog.h"
 #include "util/logging.h"
 
 #include "launcher.h"
@@ -21,6 +22,9 @@ namespace launcher {
         // therefore, subsystems need to be guarded against multiple unload attempts
         log_info("launcher", "stopping subsystems");
 
+        // before shutting down logger, dump any deferred log messages
+        deferredlogs::dump_to_logger();
+
         // flush/stop logger
         logger::stop();