Abort loading a save state if emulation isn't running.

This should actually be either !Core::IsRunning() or !Core::IsRunningAndStarted(), currently it allows you to load a save state when we're in the CORE_STOPPING state.

This is currently not a problem since the application is blocked in the CORE_STOPPING state, but PR #515 unblocks it. I'll add a commit to that PR and I'll try to find more of these instances.