RFR: Probe if Jira credentials are valid before making the actual API call

[Robin Westberg]

The expiration time calculation is unreliable, replace it with a probe.

Best regards,
Robin

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Commit messages:
 - Probe if Jira credentials are valid before making the actual API call

Changes: https://git.openjdk.java.net/skara/pull/847/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=847&range=00
  Stats: 43 lines in 5 files changed: 23 ins; 4 del; 16 mod
  Patch: https://git.openjdk.java.net/skara/pull/847.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/847/head:pull/847

PR: https://git.openjdk.java.net/skara/pull/847