RFR: 8318812: LauncherHelper.checkAndLoadMain closes jar file that's about to be re-opened [v2]

[Christian Stein]

> Please review this PR that makes the launcher helper keep a reference to the executable JAR file active after extracting the name of the main class and returning it as Class instance. Now, when loading classes from the JAR file, it hasn't to be re-opened.

Christian Stein has updated the pull request incrementally with two additional commits since the last revision:

 - Remove `String jarname` from `getMainClassFromJar`'s signature
 - Introduce and use dedicated on-close error message

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Changes:
  - all: https://git.openjdk.org/jdk/pull/17843/files
  - new: https://git.openjdk.org/jdk/pull/17843/files/bd0927d3..202a4ef4

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=17843&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=17843&range=00-01

  Stats: 81 lines in 2 files changed: 18 ins; 18 del; 45 mod
  Patch: https://git.openjdk.org/jdk/pull/17843.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17843/head:pull/17843

PR: https://git.openjdk.org/jdk/pull/17843