Better warning message for make_interp_spline size check failure

[sixpearls]

When calling make_interp_spline, if x has 2 points and k=3, the solver will pass the defensive check on line 708 but fails on line 711. It would be helpful if the corresponding error message included the possible explanation that there are not enough data points for value of k. Alternatively, we could add a specific check for this combination.