Improve error message when confusing = with == in @constraint?

[jd-foster]

Users may fairly easily confuse = with == in @constraint

cf. https://discourse.julialang.org/t/problem-setting-constraint-in-simple-lp-using-jump/123059

Minimal example

julia> function simple_confusion()
           model = Model()
           @variable(model, x)
           @variable(model, y)
           @constraint(model, x + y = 0)
end
ERROR: LoadError: MethodError: Cannot `convert` an object of type Expr to an object of type Symbol
The function `convert` exists, but no method is defined for this combination of argument types.

Is there a way to throw a more informative error that helps users debug this (likely common) mistake?