Before, `mdbook` would continue processing even when
errors, such as invalid links, are encountered. Moreover,
it would exit with a `0` return code. Such behavior is
unexpected and leads to confusion when run in CI.
Now, when links that don't point to existing files are
encounter and error is returned which yields to `mdbook`
exiting with an error code. The change in behavior has
revealed that some tests were run with invalid links.
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