Generated var shows up in error message for multi-value rule conflict

[tsandall]

In this case the error message should not include __local0__ but rather y.

> show
package repl

p.x[y] {
        y := 7
}
> p["foo"] { true }
1 error occurred: 1:1: rego_type_error: multi-value rule data.repl.p conflicts with [data.repl.p.x[__local0__]]