Make sure that javadoc and sources are mandatory for Sonatype

[jvican]

I've seen that certain releases fail to succeed because they lack javadoc and sources. I've assumed that Central forces artifacts to provide javadoc and sources, but I still need to double check this.

With the previous assumption, sbt-release-early should fail if publishArtifact in doc and sources is false and tell users that they should enable those artifacts if they want to sync to Central.

[jvican]

Some proof for this:

[info] [info]   Evaluate: javadoc-staging
[info] [info] Failed: javadoc-staging, failureMessage:Missing: no javadoc jar found in folder '/org/bitbucket/jplantdev/p2_2.11/0.2.0+1-bb317037'

But it looks like these errors do not make the whole thing fail (?).