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/CMakeLists.txt

@@ -611,6 +611,7 @@ set(SOURCE_FILES ${SOURCE_FILES}
         util/tapeled.cpp
         util/execexe.cpp
         util/dependencies.cpp
+        util/deferlog.cpp
 )
 
 source_group(TREE ${CMAKE_CURRENT_SOURCE_DIR} PREFIX "Source Files" FILES ${SOURCE_FILES})