Fix MethodError principle in MOIU/model

[odow]

Closes #1277

Now we get nice error messages

julia> MOI.set(model, MOI.ConstraintFunction(), c, MOI.SingleVariable(x))
ERROR: ArgumentError: Cannot modify functions of different types.
Constraint type is MathOptInterface.ScalarAffineFunction{Float64} while the replacement
function is of type MathOptInterface.SingleVariable.
Stacktrace:
 [1] throw_set_error_fallback(::MathOptInterface.Utilities.Model{Float64}, ::MathOptInterface.ConstraintFunction, ::MathOptInterface.ConstraintIndex{MathOptInterface.ScalarAffineFunction{Float64},MathOptInterface.EqualTo{Float64}}, ::MathOptInterface.SingleVariable; kwargs::Base.Iterators.Pairs{Union{},Union{},Tuple{},NamedTuple{(),Tuple{}}}) at /Users/oscar/.julia/dev/MathOptInterface/src/attributes.jl:1255
 [2] throw_set_error_fallback(::MathOptInterface.Utilities.Model{Float64}, ::MathOptInterface.ConstraintFunction, ::MathOptInterface.ConstraintIndex{MathOptInterface.ScalarAffineFunction{Float64},MathOptInterface.EqualTo{Float64}}, ::MathOptInterface.SingleVariable) at /Users/oscar/.julia/dev/MathOptInterface/src/attributes.jl:1255
 [3] set(::MathOptInterface.Utilities.Model{Float64}, ::MathOptInterface.ConstraintFunction, ::MathOptInterface.ConstraintIndex{MathOptInterface.ScalarAffineFunction{Float64},MathOptInterface.EqualTo{Float64}}, ::MathOptInterface.SingleVariable) at /Users/oscar/.julia/dev/MathOptInterface/src/attributes.jl:396
 [4] top-level scope at REPL[8]:1

julia> MOI.set(model, MOI.ConstraintSet(), c, MOI.LessThan(1.0))
ERROR: ArgumentError: Cannot modify sets of different types. Constraint
type is MathOptInterface.EqualTo{Float64} while the replacement set is of
type MathOptInterface.LessThan{Float64}. Use `transform` instead.
Stacktrace:
 [1] throw_set_error_fallback(::MathOptInterface.Utilities.Model{Float64}, ::MathOptInterface.ConstraintSet, ::MathOptInterface.ConstraintIndex{MathOptInterface.ScalarAffineFunction{Float64},MathOptInterface.EqualTo{Float64}}, ::MathOptInterface.LessThan{Float64}; kwargs::Base.Iterators.Pairs{Union{},Union{},Tuple{},NamedTuple{(),Tuple{}}}) at /Users/oscar/.julia/dev/MathOptInterface/src/attributes.jl:1284
 [2] throw_set_error_fallback(::MathOptInterface.Utilities.Model{Float64}, ::MathOptInterface.ConstraintSet, ::MathOptInterface.ConstraintIndex{MathOptInterface.ScalarAffineFunction{Float64},MathOptInterface.EqualTo{Float64}}, ::MathOptInterface.LessThan{Float64}) at /Users/oscar/.julia/dev/MathOptInterface/src/attributes.jl:1284
 [3] set(::MathOptInterface.Utilities.Model{Float64}, ::MathOptInterface.ConstraintSet, ::MathOptInterface.ConstraintIndex{MathOptInterface.ScalarAffineFunction{Float64},MathOptInterface.EqualTo{Float64}}, ::MathOptInterface.LessThan{Float64}) at /Users/oscar/.julia/dev/MathOptInterface/src/attributes.jl:396
 [4] top-level scope at REPL[9]:1

[blegat]

AbstractScalarSet

[blegat]

Again, I'd rather merge #1245 first :)

[odow]

Closing in favor of #1284. The diff was a pain to rebase so I made a new PR.