Include directives to missing files do not return error

[mdboom]

Our project used some include directives, that due to refactoring, inadvertently pointed to the wrong location, resulting in log error messages like the following:

[ERROR] (mdbook::preprocess::links): Error updating "{{#include ../tab_header.md}}", Could not read file for link {{#include ../tab_header.md}} (...)

These errors don't result in returning an error code from the process, so we missed them in CI.

Should these (and maybe all error log messages), result in an error code?

[ehuss]

Returning an error seems reasonable to me.