Display a user-friendly message when the JIRA server is unavailable

When the JIRA server is unreachable or in maintenance (e.g. it answers with an HTTP 503 and an HTML maintenance page), the JIRA macro displays a raw technical error box to readers, including the full query URL:

Failed to execute the JIRA macro. Cause: [Error code = [503], Error message = [Unknown error] URL = [https://jira.xwiki.org/sr/jira.issueviews:searchrequest-xml/temp/SearchRequest.xml?jqlQuery=...]]. Click on this message for details.

This was seen on xwiki.org during a jira.xwiki.org maintenance, e.g. on https://www.xwiki.org/xwiki/bin/view/ReleaseNotes/Data/XWiki/18.4.4/ where every JIRA macro of the page displayed such a box (see screenshot).

Readers can't do anything about this error and it makes the page look broken. When the server can't be reached (connection failure, timeout) or answers with a 5xx status (in particular 502/503/504), the macro should instead display a short, translatable message, for example a warning box saying "The JIRA server (jira.xwiki.org) is currently unavailable. Please try again later.", and keep the technical details (status code, URL, stack trace) in the logs or in the collapsible details.

Other errors (invalid JQL, authentication or rights problems) should keep their precise error message.