RFR: 8287971: Throw exception for missing values in .jpackage.xml

[Alexander Matveev]

- Warning will be printed if .jpackage.xml has old version number.
 - Error will be thrown if main-class and app-store values are missing from .jpackage.xml.
 - Both main-class and app-store are required values for JDK-8286850 and they might be missing if user uses app images generated by previous jpackage version.

-------------

Commit messages:
 - 8287971: Throw exception for missing values in .jpackage.xml

Changes: https://git.openjdk.java.net/jdk/pull/9098/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=9098&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8287971
  Stats: 73 lines in 6 files changed: 56 ins; 9 del; 8 mod
  Patch: https://git.openjdk.java.net/jdk/pull/9098.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/9098/head:pull/9098

PR: https://git.openjdk.java.net/jdk/pull/9098