Change config prerender.force to prerender.onError

[happycollision]

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

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Tests

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🦋 Changeset detected

Latest commit: c6f5a10

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@sveltejs/kit	Patch

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[happycollision]

All set. Thanks @benmccann!

[benmccann]

Lgtm. I left a handful of minor comments. I really like the idea of using a function to give the user more control and stole the idea for another PR #2008

[happycollision]

@benmccann , in this final pass, I took a little more liberty with the names of those properties (verb became referenceType) and also with the call sites for the error function (as I mentioned I would in my response to your request about types.

Additionally, on the types front, I created local references for types that are used more than once (or are especially long) near the top of the file. They can be inlined if that is more desirable, but I feel that things are more readable this way.