RFR: 7903732: Verify that the java executable exists

[Ludvig Janiuk]

I want to add a small check in `src/share/bin/jtreg.sh` to have jtreg see if the java it wants to run even exists, before proceeding to verify the version. This would make it easier for the user to notice tha they have mistyped the -jdk parameter.

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Commit messages:
 - 7903732 Verify that the java executable exists

Changes: https://git.openjdk.org/jtreg/pull/196/files
  Webrev: https://webrevs.openjdk.org/?repo=jtreg&pr=196&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903732
  Stats: 5 lines in 1 file changed: 5 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jtreg/pull/196.diff
  Fetch: git fetch https://git.openjdk.org/jtreg.git pull/196/head:pull/196

PR: https://git.openjdk.org/jtreg/pull/196