Warn when given an unknown ruleset tag or rule ID

[jasonkarns]

a11yCheck({}, {
  runOnly: {
    type: "tag",
    values: ["my-made-up-tag"]
  }
})

I would expect this to throw an error for my-made-up-tag not being a known ruleset tag. However, it successfully returns 0 violations. (Presumably because the tag doesn't exist, so the rule-set that was checked is empty, thus finding 0 violations.)

Additionally:

a11yCheck({}, {
  runOnly: {
    type: "rule",
    values: ["fakeRule"]
  }
})

also successfully returns 0 violations, rather than an error even though fakeRule doesn't exist.

I would expect both of these scenarios (as well as with {rules: {fakeRule: {enabled:true}}}) to return errors stating that the rule ID or tag doesn't exist.

[dylanb]

depends on #83

[WilcoFiers]

We currently have an implementation in our private repo which throws an error if this occurs. That wouldn't require the error first cb implementation.