Add more debugging info

[dinoboff]

Print the each expression evaluation result; e.g., with chai:

2) A valid set of security rules and data can have write errors:
   AssertionError: Expected a user authenticated via Password Login to be able to write true to users/password:500f6e96-92c6-4f60-ad5d-207253aee4d3/innocent, but the rules denied the write.
    /users/password:500f6e96-92c6-4f60-ad5d-207253aee4d3: write "auth !== null && root.child('users').child(auth.uid).child('king').val() === true"  => false
      (
        auth !== null  [=> true]
        && root.child("users").child(auth.uid).child("king").val() === true  [=> false]
      )  [=> false]
      using [
        auth = {"uid":"password:500f6e96-92c6-4f60-ad5d-207253aee4d3","id":1,"provider":"password"}
        auth.uid = "password:500f6e96-92c6-4f60-ad5d-207253aee4d3"
        root = {"path":"","exists":true}
        root.child('users') = {"path":"users","exists":true}
        root.child('users').child(auth.uid) = {"path":"users/password:500f6e96-92c6-4f60-ad5d-207253aee4d3","exists":true}
        root.child('users').child(auth.uid).child('king') = {"path":"users/password:500f6e96-92c6-4f60-ad5d-207253aee4d3/king","exists":false}
        root.child('users').child(auth.uid).child('king').val() = null
      ]

    /users/password:500f6e96-92c6-4f60-ad5d-207253aee4d3/innocent: validate "data.parent().child('on-fire').val() === false"  => false
      data.parent().child("on-fire").val() === false  [=> false]
      using [
        data = {"path":"users/password:500f6e96-92c6-4f60-ad5d-207253aee4d3/innocent","exists":false}
        data.parent() = {"path":"users/password:500f6e96-92c6-4f60-ad5d-207253aee4d3","exists":true}
        data.parent().child('on-fire') = {"path":"users/password:500f6e96-92c6-4f60-ad5d-207253aee4d3/on-fire","exists":false}
        data.parent().child('on-fire').val() = null
      ]

    No .write rule allowed the operation.
    One or more .validate rules disallowed the operation.
    write was denied.
    at Assertion.<anonymous> (plugins/chai.js:98:12)
    at Assertion.ctx.(anonymous function) [as path] (node_modules/chai/lib/chai/utils/addMethod.js:41:25)
    at Context.<anonymous> (docs/chai/examples/failing.js:25:9)