4.x: Add "fail on warning" for javadoc and fix all issues

[tomas-langer]

Sample project: codegen

Requires the following in pom.xml:

    <properties>
        <javadoc.fail-on-warnings>true</javadoc.fail-on-warnings>
    </properties>

This will cause javadoc to fail on any warning. This is the desired state of Helidon - we should fix all warnings reported.
This can be done module by module (or it can be defined on a project module to trigger it for all child modules).