ruleBase.addPackage(packageBuilder.getPackage()); should fail-fast with a clear error message if the packageBuilder has any errors

After you 've build your package with packageBuilder,
you've either called the hasErrors() method and done something about it
or you didn't check the hasErrors() method because you didn't know (or want to know) it existed and you expect it to throw an exception because hasErrors() returns true.

The principle of least surpise. You don't expect that it continues like everything is cool (while it's not) and crashes the moment you fire your first rules.

The exception should be a runtime exception and the message should at least mention the first of the errors.
The exception should probably contain a list of all errors of course too as a property.
See InvalidStateException in hibernate-validator.