RFR: 1686: Warning message for backport mistakes is not clear

[Zhao Song]

A user reported that he found many PR with "issue is not open" warning. After discussing in slack, we found that this happens because backport-style PR wasn't used. 

To improve the experience of user, we need to give some useful information to the user instead of just telling them "Issue is not open".

In this patch, when a user is trying to create a PR related with a resolved issue, we will assume that the user wants to create a backport PR but in the wrong way. So in this case, we will tell the user how to create a correct backport pr. In other case, for example, the issue is closed with "Not an Issue", we will still just tell the user "Issue is not open".

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Commit messages:
 - SKARA-1686

Changes: https://git.openjdk.org/skara/pull/1425/files
 Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1425&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-1686
  Stats: 48 lines in 2 files changed: 44 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/skara/pull/1425.diff
  Fetch: git fetch https://git.openjdk.org/skara pull/1425/head:pull/1425

PR: https://git.openjdk.org/skara/pull/1425