cleanup error-handling in SDL_egl.c - always set error message in SDL_EGL_ChooseConfig / SDL_EGL_CreateContext - assume SDL_EGL_DeleteContext does not alter the error message - sync generic error message of SDL_EGL_MakeCurrent with SDL_EGL_Get/SetSwapInterval - do not overwrite error message of SDL_EGL_ChooseConfig in WINRT_CreateWindow