Fallback to default client configuration if `requestConfigFromFrame` configuration is invalid

The Hypothesis white-label VitalSource book reader at https://hypothesis.vitalsource.com is configured for use inside the Hypothesis LMS app. To enable the Hypothesis client to talk to our LMS app, the book reader frame that loads the client sets the `requestConfigFromFrame` key:

```js
// `window.hypothesisConfig` function in the iframe where the client is loaded
function () {return {requestConfigFromFrame: {origin: "https://lms.hypothes.is",ancestorLevel: 3},openSidebar: true};} = $1
```

If you try to access a book at https://hypothesis.vitalsource.com directly, the client fails to load due to this configuration. It logs this error on startup:

The preferred solution here would be that https://hypothesis.vitalsource.com changes the Hypothesis client configuration depending on whether it was launched by our LMS app or not:

1. When launched by our LMS app, the H-VS book reader should use the above configuration
2. Otherwise, the H-VS reader should omit the `requestConfigFromFrame` key

As a fallback we can implement purely on our side, ignore the `requestConfigFromFrame` configuration with a console warning if it is invalid (eg. specifies that the configuration should be retrieved from the ancestor N levels up the frame tree, and N is greater than the number of ancestors). The console warning is important in case the code path ever ends up getting executed when it shouldn't be.