RFR: 8268023: Improve diagnostic for HandshakeFailureTest

[Daniel Fuchs]

Please find below a trivial change to improve diagnostic in case of failure in the HandshakeFailureTest.
The AssertionError observed will have more information next time it fires, and the stack trace will appear as the cause of the runtime exception thrown by the test.

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Commit messages:
 - 8268023: Improve diagnostic for HandshakeFailureTest

Changes: https://git.openjdk.java.net/jdk/pull/4281/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4281&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8268023
  Stats: 6 lines in 2 files changed: 2 ins; 0 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4281.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4281/head:pull/4281

PR: https://git.openjdk.java.net/jdk/pull/4281