javadoc_test CI build does not run mvn javadoc target

[andreamlin]

https://circleci.com/gh/GoogleCloudPlatform/google-cloud-java/8882?utm_campaign=vcs-integration-link&utm_medium=referral&utm_source=github-build-link should have failed on making the javadoc (#3459 fixes the javadoc error)

We should ensure that the CI build for javadoc_test does indeed run the javadoc target and throw an exception if there's an error.

This is an annoying issue for when google-cloud-java is being released, and the javadoc target is actually run.