RFR: CODETOOLS-7903325: jtreg check for macos version should take into account the process exit code

[Jonathan Gibbons]

Please review a simple change to improve the reporting in the check for the macOS version.

Tested by temporarily modifying the code in place to change the command or its exit code.

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Commit messages:
 - CODETOOLS-7903325: jtreg check for macos version should take into account the process exit code

Changes: https://git.openjdk.org/jtreg/pull/124/files
 Webrev: https://webrevs.openjdk.org/?repo=jtreg&pr=124&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903325
  Stats: 10 lines in 1 file changed: 8 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jtreg/pull/124.diff
  Fetch: git fetch https://git.openjdk.org/jtreg pull/124/head:pull/124

PR: https://git.openjdk.org/jtreg/pull/124