RFR: 8282467: add extra diagnostics for JDK-8268184

[Dean Long]

Add more info to the assert to help diagnose what went wrong the next time it fails.

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Commit messages:
 - add extra info to assert

Changes: https://git.openjdk.java.net/jdk/pull/7642/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7642&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8282467
  Stats: 4 lines in 1 file changed: 2 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7642.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7642/head:pull/7642

PR: https://git.openjdk.java.net/jdk/pull/7642