Adding a knob that can't be added (i.e it's already on another node) doesn't fail gracefully

```
g = nuke.createNode('Grade')

k = nuke.String_Knob('one')
l = nuke.String_Knob('two')
m = nuke.String_Knob('three')

g.addKnob(k)
g.addKnob(l)
g.addKnob(m)

knobwrangler.insert(m, g, 'one')
```

You get a RuntimeError, and depending on the state of your insert point, knob names/labels are changed or knobs are lost and not recovered. Shouldn't fail this badly on an invalid change.