Upgrade book linkcheck warnings to errors

AzureMarker · AzureMarker · commit b0fa1a5811f1 · 2020-05-11T16:54:26.000-04:00
diff --git a/book/book.toml b/book/book.toml
@@ -14,6 +14,7 @@ additional-js = ["mermaid.min.js", "mermaid-init.js"]
 
 [output.linkcheck]
 follow-web-links = true
+warning-policy = "error"
 
 [preprocessor.toc]
 command = "mdbook-toc"
