fixes #1940

Essentially, we are doing this:

kit: {
  prerender: {
    crawl: boolean;
    enabled: boolean;
-   force: boolean;
+   onError: 'fail' | 'continue' | ((details: RequestDetails) => void | never);
    pages: string[];
  }
}

• Changing from force to onError gives a somewhat more descriptive
  name to the option.
• Using the strings "fail" and "continue" further clarifies what the
  option does.
• Providing your own function to deal with an error allows for
  fine-tuning which errors fail the build and which do not.

The original ticket suggested that the custom function return a boolean,
but after seeing the implementation of the error handler in svelteKit, I
thought it more fitting to just allow it the exact same API: throw if
you want the build to fail.

Teaching the new option via error message:

This PR also adds a failing option check for using the old force property, and gives you the proper value to use in its place as well. This should help when people version bump to the latest build.

Examples of various acceptable options, and the output when a link is broken

The new handler function option

Throw to fail

Anything but throwing will continue the build

The old behavior via the new settings

fail

continue

Before submitting the PR, please make sure you do the following

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• Ideally, include a test that fails without this PR but passes with it.

Tests

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