Show term name in only-one-level error

[jtrakk]

ArgumentError: only one level found: foo (need at least two to compute contrasts).
Stacktrace:
  [1] StatsModels.ContrastsMatrix(contrasts::DummyCoding, levels::Vector{Int64})
    @ StatsModels ~/.julia/packages/StatsModels/MDeyQ/src/contrasts.jl:185

it would be helpful if it would say which column/term the problem is in.

[kleinschmidt]

I agree that would be nice to have. It's currently a bit tricky since you'll see that it's the ContrastsMatrix constructor that does that check and throws the error, and that doesn't know about the context in which it's being called (including the term name). So we'd have to catch and re-throw the error at the term construction site (probably concrete_term if memory serves) but that seems a bit icky to me somehow.

[kleinschmidt]

(I will say that the idea with printing the actual level it did find is to help track down which column it is that caused the problem, even though giving the column name would be even better)