RFR(XS): 8218581: Incorrect exception message generation

[Mikael Vidstedt]

Please review this small change which fixes the generation of the exception message when defining modules with disallowed packages.

Bug: https://bugs.openjdk.java.net/browse/JDK-8218581 <https://bugs.openjdk.java.net/browse/JDK-8218581>
Webrev: http://cr.openjdk.java.net/~mikael/webrevs/8218581/webrev.00/open/webrev/ <http://cr.openjdk.java.net/~mikael/webrevs/8218581/webrev.00/open/webrev/>

I’ll run tier1 on this to make sure it still builds, and I have manually verified that the expected exception message is (still) generated.

Cheers,
Mikael