Fix errors/warnings when generating javadoc

[kaklakariada]

Generating javadoc currently fails because of missing tags.

1. Set failOnError and failOnWarnings to true in pom.xml
2. Run mvn javadoc:javadoc
3. Fix errors/warnings
4. Add a build breaker for CI builds to avoid this situation in future releases.

[redcatbear]

Closed with #199.