[Utilities] fix unsupported get in model.jl

[odow]

Closes #2005

[blegat]

Wasn't it already fixed by #2007 ?

[odow]

That fixed part of the problem. But we needed this as well.

[blegat]

Getting a MethodError is the right behavior so I don't thing there is anything to fix. For non-variable constraints, this is also what you get. We could have a default fallback in src/attributes if we want to replace the MethodError by something else. Or we could say that all implementation should do it explicitly but then we need to update all of them in MOI.Utilities at least to have something consistent (and tests as well would be even better)