Show proper error for unsupported mappings (8-Bit and 16-Bit)

[georgwiese]

When activating a mapping on a 16-bit segmentation, it is correctly applied in the "segmentation" tab, but not in the viewports:

Here you can see how the two segments at current position and mouse position are both mapped to ID 1, but they are rendered in different colors.

I wasn't able to repreduce this on a 32-Bit segmentation.

[philippotto]

See #2738 ;) Is this important for your workflow, @georgwiese?

[georgwiese]

Ah, I see :)

Actually, it's not important, because we can just use 32-Bit segmentations. That makes sense for us anyway because 2¹⁶ isn't a lot. For my purpose, it would be fine if WK didn't support it, but it shouldn't fail silently.

[philippotto]

Ok, cool, then I'll change this issue to cover proper error reporting and leave the other issue at low-pri.