Polished up the global sprite sheet system.

Moved the global sprite sheet instance to the Sprites namespace. Also added src/error.hpp, which will be used to maintain error codes.  Began adding error codes for the global sprite sheet, however there are other areas that likely should be updated to report some sort of error in certain circumstances.
Fixed alignment of Error_code enum in error.hpp and fixed printing of an error code in visible_game_object.cpp.