RFR: 2003: Timing out lock in RestRequestCache stops the bot

[Zhao Song]

As Erik said in the issue description, it's better to throw an exception here.

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Commit messages:
 - SKARA-2003

Changes: https://git.openjdk.org/skara/pull/1555/files
 Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1555&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-2003
  Stats: 3 lines in 1 file changed: 0 ins; 1 del; 2 mod
  Patch: https://git.openjdk.org/skara/pull/1555.diff
  Fetch: git fetch https://git.openjdk.org/skara.git pull/1555/head:pull/1555

PR: https://git.openjdk.org/skara/pull/1555