Improve error message when `opBinaryRight` is missing

[Bolpat]

This is the error message you get on a missing opBinaryRight:

onlineapp.d(17): Error: operator `/` is not defined for type `T`
onlineapp.d(6):        perhaps overload the operator with `auto opBinaryRight(string op : "/")(int rhs) {}`

The suggested parameter should be named lhs.