Make it more obvious that one-jar should be run after package

[erikkaplun]

If I understand correctly how sbt-onejar operates then one needs to run the package command first to generate all the "normal" JARs, and only then run the one-jar command to have those compiled into a big fat JAR with the dependencies. However, if the package command hasn't been run previously, one-jar will still happily produce a non-working JAR with just the dependencies in it. I think this serves no value and should be fixed either by automatically running package or displaying an error message.