Constraint macro with single = throws uninformative error message

[odow]

From slack:

julia> @constraint(model, x = 1)
ERROR: LoadError: At REPL[29]:1: `@constraint(model, x = 1)`: Not enough positional arguments
Stacktrace:
 [1] error(::String, ::String)
   @ Base ./error.jl:42
 [2] _macro_error(macroname::Symbol, args::Tuple{Symbol, Expr}, source::LineNumberNode, str::String)
   @ JuMP ~/.julia/packages/JuMP/puvTM/src/macros.jl:1597
 [3] (::JuMP.var"#_error#100"{Tuple{Symbol, Expr}, Symbol, LineNumberNode})(str::String)
   @ JuMP ~/.julia/packages/JuMP/puvTM/src/macros.jl:745
 [4] _constraint_macro(args::Tuple{Symbol, Expr}, macro_name::Symbol, parsefun::typeof(parse_constraint), source::LineNumberNode)
   @ JuMP ~/.julia/packages/JuMP/puvTM/src/macros.jl:753
 [5] var"@constraint"(__source__::LineNumberNode, __module__::Module, args::Vararg{Any, N} where N)
   @ JuMP ~/.julia/packages/JuMP/puvTM/src/macros.jl:926
in expression starting at REPL[29]:1

julia> y = 1
1

julia> @constraint(model, x = y)
ERROR: LoadError: At REPL[31]:1: `@constraint(model, x = y)`: Not enough positional arguments
Stacktrace:
 [1] error(::String, ::String)
   @ Base ./error.jl:42
 [2] _macro_error(macroname::Symbol, args::Tuple{Symbol, Expr}, source::LineNumberNode, str::String)
   @ JuMP ~/.julia/packages/JuMP/puvTM/src/macros.jl:1597
 [3] (::JuMP.var"#_error#100"{Tuple{Symbol, Expr}, Symbol, LineNumberNode})(str::String)
   @ JuMP ~/.julia/packages/JuMP/puvTM/src/macros.jl:745
 [4] _constraint_macro(args::Tuple{Symbol, Expr}, macro_name::Symbol, parsefun::typeof(parse_constraint), source::LineNumberNode)
   @ JuMP ~/.julia/packages/JuMP/puvTM/src/macros.jl:753
 [5] var"@constraint"(__source__::LineNumberNode, __module__::Module, args::Vararg{Any, N} where N)
   @ JuMP ~/.julia/packages/JuMP/puvTM/src/macros.jl:926
in expression starting at REPL[31]:1

julia> @constraint(model, 2x + 1 = y)
ERROR: LoadError: At REPL[32]:1: `@constraint(model, 2x + 1 = begin
        #= REPL[32]:1 =#
        y
    end)`: Not enough positional arguments
Stacktrace:
 [1] error(::String, ::String)
   @ Base ./error.jl:42
 [2] _macro_error(macroname::Symbol, args::Tuple{Symbol, Expr}, source::LineNumberNode, str::String)
   @ JuMP ~/.julia/packages/JuMP/puvTM/src/macros.jl:1597
 [3] (::JuMP.var"#_error#100"{Tuple{Symbol, Expr}, Symbol, LineNumberNode})(str::String)
   @ JuMP ~/.julia/packages/JuMP/puvTM/src/macros.jl:745
 [4] _constraint_macro(args::Tuple{Symbol, Expr}, macro_name::Symbol, parsefun::typeof(parse_constraint), source::LineNumberNode)
   @ JuMP ~/.julia/packages/JuMP/puvTM/src/macros.jl:753
 [5] var"@constraint"(__source__::LineNumberNode, __module__::Module, args::Vararg{Any, N} where N)
   @ JuMP ~/.julia/packages/JuMP/puvTM/src/macros.jl:926
in expression starting at REPL[32]:1