RFR: jira: make links() more reliable

[Erik Helin]

Hi all,

please review this patch that makes `JiraIssue.links()` a bit more reliable in the face of unexpected HTTP codes. I
also took the liberty to use an `ArrayList` instead of concatenating `Stream`s.

Thanks,
Erik

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Commit messages:
 - jira: make links() more reliable

Changes: https://git.openjdk.java.net/skara/pull/810/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=810&range=00
  Stats: 19 lines in 1 file changed: 7 ins; 0 del; 12 mod
  Patch: https://git.openjdk.java.net/skara/pull/810.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/810/head:pull/810

PR: https://git.openjdk.java.net/skara/pull/810