Throw GetAttributeNotAllowed in AbstractBridge getter

[blegat]

If the user sets cannot_unbridge = true in MOI.Bridges.runtests then it is allowed to not implement MOI.ConstraintSet and MOI.ConstraintFunction (since these are actually mostly useless to implement).
This was working for SetMapBridge thanks to

MathOptInterface.jl/src/Bridges/Constraint/set_map.jl

Lines 116 to 119 in f604fc1

	if err isa MOI.Bridges.MapNotInvertible
	msg = _not_invertible_error_message(attr, err.message)
	throw(MOI.GetAttributeNotAllowed(attr, msg))
	end

but actually not for classical AbstractBridge because it was throwing an ArgumentError. This PR fixes this.