Print more information when user define `BinaryQuadraticModel` on the RHS of an equation

[mhramani]

More information is needed when user define constraints that has BinaryQuadraticModel as RHS

x = dimod.Binary('x')
y = dimod.Binary('y')
cqm = dimod.CQM()
cqm.add_constraint(x <= y)


TypeError: '<=' not supported between instances of 'BinaryQuadraticModel' and 'BinaryQuadraticModel'

From this error it is not clear that RHS should only be constant.