Improve UnsupportedConstraint error

src/constraints.jl

@@ -26,21 +26,47 @@ function supports_constraint(
 end
 
 """
-    struct UnsupportedConstraint{F<:AbstractFunction, S<:AbstractSet} <: UnsupportedError
-        message::String # Human-friendly explanation why the attribute cannot be set
+    struct UnsupportedConstraint{F<:AbstractFunction,S<:AbstractSet} <: UnsupportedError
+        message::String
     end
 
 An error indicating that constraints of type `F`-in-`S` are not supported by
 the model, that is, that [`supports_constraint`](@ref) returns `false`.
+
+```jldoctest
+julia> import MathOptInterface as MOI
+
+julia> showerror(stdout, MOI.UnsupportedConstraint{MOI.VariableIndex,MOI.ZeroOne}())
+UnsupportedConstraint: `MathOptInterface.VariableIndex`-in-`MathOptInterface.ZeroOne` constraints are not supported by the
+solver you have chosen, and we could not reformulate your model into a
+form that is supported.
+
+To fix this error you must choose a different solver.
+
+```
 """
 struct UnsupportedConstraint{F<:AbstractFunction,S<:AbstractSet} <:
        UnsupportedError
-    message::String # Human-friendly explanation why the attribute cannot be set
+    # Human-friendly explanation why the attribute cannot be set
+    message::String
 end
+
 UnsupportedConstraint{F,S}() where {F,S} = UnsupportedConstraint{F,S}("")
 
-function element_name(::UnsupportedConstraint{F,S}) where {F,S}
-    return "`$F`-in-`$S` constraint"
+function Base.showerror(io::IO, err::UnsupportedConstraint{F,S}) where {F,S}
+    print(
+        io,
+        """
+        UnsupportedConstraint: `$F`-in-`$S` constraints are not supported by the
+        solver you have chosen, and we could not reformulate your model into a
+        form that is supported.
+
+        To fix this error you must choose a different solver.
+
+        $(err.message)
+        """,
+    )
+    return
 end
 
 """

test/errors.jl

@@ -46,26 +46,29 @@ function test_errors_inconsistent_vectorscalar()
     return
 end
 
-function _test_errors_UnsupportedConstraint(f)
+function test_errors_UnsupportedConstraint()
     model = DummyModel()
     vi = MOI.VariableIndex(1)
-    @test_throws(MOI.UnsupportedConstraint, f(model, vi, MOI.EqualTo(0)),)
+    @test_throws(
+        MOI.UnsupportedConstraint,
+        MOI.add_constraint(model, vi, MOI.EqualTo(0)),
+    )
+    msg = """
+    UnsupportedConstraint: `$(MOI.VariableIndex)`-in-`$(MOI.EqualTo{Int})` constraints are not supported by the
+    solver you have chosen, and we could not reformulate your model into a
+    form that is supported.
+
+    To fix this error you must choose a different solver.
+
+    """
     try
-        f(model, vi, MOI.EqualTo(0))
+        MOI.add_constraint(model, vi, MOI.EqualTo(0))
     catch err
-        @test sprint(showerror, err) ==
-              "$(MOI.UnsupportedConstraint{MOI.VariableIndex,MOI.EqualTo{Int}}):" *
-              " `$MOI.VariableIndex`-in-`$MOI.EqualTo{$Int}` constraint is" *
-              " not supported by the model."
+        @test sprint(showerror, err) == msg
     end
     return
 end
 
-function test_errors_UnsupportedConstraint()
-    _test_errors_UnsupportedConstraint(MOI.add_constraint)
-    return
-end
-
 function test_errors_UnsupportedConstraint_shortcut()
     model = DummyModel()
     vi = MOI.VariableIndex(1)