RFR: 939: Improve robustness and error reporting on invalid fixVersion

[Robin Westberg]

If a fixVersion isn't found, ensure that we refresh the cache and log a proper error message if it still isn't found.

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Commit messages:
 - Improve robustness and error reporting on invalid fixVersion

Changes: https://git.openjdk.java.net/skara/pull/1099/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=1099&range=00
  Issue: https://bugs.openjdk.java.net/browse/SKARA-939
  Stats: 17 lines in 2 files changed: 15 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/skara/pull/1099.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/1099/head:pull/1099

PR: https://git.openjdk.java.net/skara/pull/1099