When using 
publishHTML with allowMissing: false and directory is not found, publisher sets the build status to FAILURE, but does not fail the build.
 
Seems issue is in src/main/java/htmlpublisher/HtmlPublisher.java
 
where the following returns true and not false.
 
if (!archiveDir.exists()) {
    listener.error("Specified HTML directory '" + archiveDir + "' does not exist.");
    if (!allowMissing)

{         build.setResult(Result.FAILURE);         return true;     }

}