RFR: 8337163: Improve SA error message when failing to attach to a core file

[Chris Plummer]

When SA fails to open/attach to a core file on linux or OSX, include a note about using LIBSAPROC_DEBUG=1 in the error message. No one remembers this magic, so it is useful to provide here. Even I always have to resort to looking up the symbol in the SA source since I can never remember it. The extra output you get with LIBSAPROC_DEBUG=1 is often enough for the user to determine the root cause, or at least help an SA engineer determine the cause.

Test by running all SA tests and also tier1.

Also did manual testing with forced errors reading core files to make sure the error output is as expected.

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Commit messages:
 - Better error message when failing to read core file.

Changes: https://git.openjdk.org/jdk/pull/20800/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20800&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8337163
  Stats: 4 lines in 2 files changed: 0 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/20800.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20800/head:pull/20800

PR: https://git.openjdk.org/jdk/pull/20800