Improve text of showerror for NotAllowedError

[odow]

Closes #2756

I now get

julia> import MathOptInterface as MOI

julia> begin
           struct Model2756 <: MOI.ModelLike
               variables::Vector{Float64}
               Model2756() = new(Float64[])
           end
           MOI.is_empty(model::Model2756) = isempty(model.variables)
           MOI.empty!(model::Model2756) = (empty!(model.variables); nothing)
           MOI.supports_incremental_interface(::Model2756) = true
           function MOI.supports_constraint(
               ::Model2756,
               ::Type{MOI.VariableIndex},
               ::Type{MOI.GreaterThan{Float64}},
           )
               return true
           end
           function MOI.add_variable(model::Model2756)
               push!(model.variables, -Inf)
               return MOI.VariableIndex(length(model.variables))
           end
           function MOI.add_constraint(
               model::Model2756,
               x::F,
               s::S,
           ) where {F<:MOI.VariableIndex,S<:MOI.GreaterThan}
               model.variables[x.value] = s.lower
               return MOI.ConstraintIndex{F,S}(x.value)
           end
           model = MOI.instantiate(Model2756; with_bridge_type = Float64)
           x = MOI.add_variables(model, 2)
           MOI.add_constraint(model, MOI.VectorOfVariables(x), MOI.Nonnegatives(2))
           MOI.Bridges.print_active_bridges(model)
       end
ERROR: MathOptInterface.GetAttributeNotAllowed{MathOptInterface.ObjectiveFunctionType}:

## Cause

Getting attribute MathOptInterface.ObjectiveFunctionType() cannot be performed because:

Model2756 does not support getting the attribute MathOptInterface.ObjectiveFunctionType().

## Fixing this error

An `MOI.NotAllowedError` error occurs when you have tried to do something that
is not implemented by the solver.

The most common way to fix this error is to wrap the optimizer in a
`MOI.Utilities.CachingOptimizer`.

For example, if you are using `JuMP.Model` or `JuMP.set_optimizer`, do:
```julia
model = JuMP.Model(optimizer; with_cache_type = Float64)
model = JuMP.GenericModel{T}(optimizer; with_cache_type = T)
JuMP.set_optimizer(model, optimizer; with_cache_type = Float64)
```
Similarly, if you are using `MOI.instantiate`, do:
```julia
model = MOI.instantiate(optimizer; with_cache_type = Float64)
```

Stacktrace:
 [1] get_fallback(model::Model2756, attr::MathOptInterface.ObjectiveFunctionType)
   @ MathOptInterface ~/.julia/dev/MathOptInterface/src/attributes.jl:409
 [2] get(::Model2756, ::MathOptInterface.ObjectiveFunctionType)
   @ MathOptInterface ~/.julia/dev/MathOptInterface/src/attributes.jl:390
 [3] get
   @ ~/.julia/dev/MathOptInterface/src/Bridges/bridge_optimizer.jl:1177 [inlined]
 [4] print_active_bridges(io::Base.TTY, b::MathOptInterface.Bridges.LazyBridgeOptimizer{Model2756})
   @ MathOptInterface.Bridges ~/.julia/dev/MathOptInterface/src/Bridges/debug.jl:544
 [5] print_active_bridges(::MathOptInterface.Bridges.LazyBridgeOptimizer{Model2756})
   @ MathOptInterface.Bridges ~/.julia/dev/MathOptInterface/src/Bridges/debug.jl:556
 [6] top-level scope
   @ REPL[42]:31

[blegat]

Couldn't be clearer, I like it