Made some changes from review comments:

- Made LoadKernelSystemMode return a pair consisting of a system mode and a result code (Could use review).
- Deleted ErrorOpenGL error code in favor of just having ErrorVideoCore.
- Made dialog messages more clear.
- Compared archive ID in fs_user.cpp to ArchiveIdCode::NCCH as opposed to hex magic.
- Cleaned up some other stuff.

src/core/core.h

@@ -43,7 +43,6 @@ public:
         ErrorSystemFiles,               ///< Error in finding system files
         ErrorSharedFont,                ///< Error in finding shared font
         ErrorVideoCore,                 ///< Error in the video core
-        ErrorOpenGL,                    ///< Error when initializing OpenGL
         ErrorUnknown                    ///< Any other error
     };