Fix: Correct [object Object] output of error.data.

[jrpool]

Stops error.data from being output as [object Object] when the data property
is an object, such as a rule’s options object.

What is the purpose of this pull request? (put an "X" next to item)

[ ] Documentation update
[x] Bug fix (template)
[ ] New rule (template)
[ ] Changes an existing rule (template)
[ ] Add autofixing to a rule
[ ] Add a CLI option
[ ] Add something to the core
[ ] Other, please explain:

Tell us about your environment

• ESLint Version:
  4.10.0
• Node Version:
  9.0.0
• npm Version:
  5.5.1
  What parser (default, Babel-ESLint, etc.) are you using?
  default
  Please show your full configuration:

"use strict";

const path = require("path");
const rulesDirPlugin = require("eslint-plugin-rulesdir");

rulesDirPlugin.RULES_DIR = path.join(__dirname, "tools/internal-rules");

module.exports = {
    root: true,
    plugins: [
        "eslint-plugin",
        "node",
        "rulesdir"
    ],
    extends: [
        "./packages/eslint-config-eslint/default.yml",
        "plugin:node/recommended",
        "plugin:eslint-plugin/recommended"
    ],
    rules: {
        "eslint-plugin/consistent-output": "error",
        "eslint-plugin/no-deprecated-context-methods": "error",
        "eslint-plugin/prefer-output-null": "error",
        "eslint-plugin/prefer-placeholders": "error",
        "eslint-plugin/report-message-format": ["error", "[^a-z].*\\.$"],
        "eslint-plugin/test-case-property-ordering": "error",
        "eslint-plugin/test-case-shorthand-strings": "error",
        "rulesdir/multiline-comment-style": "error"
    }
};

What did you do? Please include the actual source code causing the issue.
In a test file in test/lib/rules, in an invalidity test, set the value of options to an array of 1 option, which contained a property whose name was not specified in the schema of the rule. For example, set line 398 of test/lib/rules/object-property-newline.js to:
options: [{ XallowMultiplePropertiesPerLineX: true }],
What did you expect to happen?
I expected an intelligible error message explaining what was wrong.
What actually happened? Please include the actual, raw output from ESLint.
I received an error message that did not sufficiently explain what was wrong, namely:
Value [object Object] should NOT have additional properties.

What changes did you make? (Give an overview)

1. I modified config-validator to check for the type of error.data. If error.data has a type other than string and is not an array, I changed it to its JSON stringification.
2. I modified the expected error messages of 2 config-validator tests to match the formats of error.data that arise from their JSON stringification.

Is there anything you'd like reviewers to focus on?
Would it be better to check for other types not yet in use, or to invert the check and check only for non-array objects, or to JSON stringify everything regardless of type?

[mysticatea]

Thank you for the contribution!

Looks great to me.
Personally, I think better if it JSON-stringify everything regardless of type.

[jrpool]

Thank you for your comment, @mysticatea. I’ll be happy to make that change, unless anybody else disagrees.

[platinumazure]

With the error.data no longer surrounded by quotation marks, I would also be okay with unconditional stringification. Thanks!

[jrpool]

I have revised this PR to JSON-stringify error.data in all cases, as recommended by @mysticatea and @platinumazure.

[mysticatea]

LGTM, thank you!