The first and last knots (which are often also poles) of a B-Spline are not
represented in sketcher as knots, but as end points of the curve. This creates a
problem when we want to change the multiplicities of those knots. As a
workaround, these points are treated differently.
NOTE: While it is technically possible to have end knots in a non-periodic
spline with degree <= degree of the curve, `Geom_BSplineCurve::RemoveKnot()`
fails for the first and last knots, so we are stuck with this. So, this commit
effectively only makes knot multiplicity change possible for periodic splines.
For non-periodic splines this commit just changes the confusing "no knot is
selected" message to an unhelpful "multiplicity modification failed".
3e7830c21b