Speed error indicator

Today I've fixed the steering of the speed error indicator in landing mode (the little vertical line on the velocity vector). The function has been there since before, but now I've also got the logic for it. The code is written for both 12 degrees and 15.5 degrees alpha, but since we haven't connected the button to change between these, only 12 degrees work at this moment. I've also fixed the warning for critically low speed.