RFR: 8337299: vmTestbase/nsk/jdb/stop_at/stop_at002/stop_at002.java failure goes undetected

[Chris Plummer]

The test is setting breakpoints on the wrong line numbers, which causes setting up the breakpoint to fail, but the test also has buggy error checking, so the test doesn't detect the failures and still passes. I fixed the breakpoint line numbers and the error checking. More details in the first comment.

Testing tbd: I'll run the tier5 svc testing, which includes this test suite.

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Commit messages:
 - fix breakpoint line numbers. fix test error checking

Changes: https://git.openjdk.org/jdk/pull/20366/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20366&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8337299
  Stats: 60 lines in 2 files changed: 40 ins; 8 del; 12 mod
  Patch: https://git.openjdk.org/jdk/pull/20366.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20366/head:pull/20366

PR: https://git.openjdk.org/jdk/pull/20366