Improve error message when = is used instead of ==

[odow]

Closes #3891

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

julia> function quantile_regression_linear()
           # retrieve views of the relevant data 
           y = rand(1000)
           x = rand(1000)
           N = length(y)
           # initialize the optimization model and solve it 
           model = Model()
           set_silent(model)
           @variable(model, u_plus[1:N] ≥ 0.0)
           @variable(model, u_minus[1:N] ≤ 0.0)
           @variable(model, θ[1:2])
           @constraint(model, [i in 1:N], y[i] - θ[1] - θ[2]*x[i] = u_plus[i] + u_minus[i])
       end
ERROR: LoadError: At REPL[11]:12: `@constraint(model, [i in 1:N], (y[i] - θ[1]) - θ[2] * x[i] = begin
        #= REPL[11]:12 =#
        u_plus[i] + u_minus[i]
    end)`: Invalid keyword argument detected. If you are trying to construct an equality constraint, use `==` instead of `=`.
Stacktrace:
 [1] error(::String, ::String)
   @ Base ./error.jl:44
 [2] (::JuMP.Containers.var"#error_fn#98"{String})(str::String)
   @ JuMP.Containers ~/.julia/dev/JuMP/src/Containers/macro.jl:331
 [3] parse_macro_arguments(error_fn::JuMP.Containers.var"#error_fn#98"{…}, args::Tuple{…}; valid_kwargs::Nothing, num_positional_args::Nothing)
   @ JuMP.Containers ~/.julia/dev/JuMP/src/Containers/macro.jl:58
 [4] parse_macro_arguments(error_fn::Function, args::Tuple{Symbol, Expr, Expr})
   @ JuMP.Containers ~/.julia/dev/JuMP/src/Containers/macro.jl:41
 [5] var"@constraint"(__source__::LineNumberNode, __module__::Module, input_args::Vararg{Any})
   @ JuMP ~/.julia/dev/JuMP/src/macros/@constraint.jl:103
in expression starting at REPL[11]:12
Some type information was truncated. Use `show(err)` to see complete types.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.58%. Comparing base (1230ea6) to head (a89a34a).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3892   +/-   ##
=======================================
  Coverage   99.58%   99.58%           
=======================================
  Files          43       43           
  Lines        6041     6043    +2     
=======================================
+ Hits         6016     6018    +2     
  Misses         25       25           

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