RFR: JDK-8221101: Update JNLPConverter to support latest jpackage CLI changes

[Alexander Matveev]

Please review the jpackage fix for bug [1] at [2].

This is a fix for the JDK-8200758-branch branch of the open sandbox 
repository (jpackage).

- Updated JNLPConverter to support latest CLI changes and bug fixes.
- Added getMessage() to exceptions, otherwise null was printed.
- Restored characters() in XMLParser.java, otherwise some JNLP files 
were not parsed correctly. This function was deleted during code cleanup.

[1] https://bugs.openjdk.java.net/browse/JDK-8221101

[2] http://cr.openjdk.java.net/~almatvee/8221101/webrev.00/

Thanks,
Alexander