While fixing this, it would be good to make the error reporting of the macro generic enough to also cover invalid parameter values, which is criterion E2 of the WYSIWYG usability audit of the XWiki macros: an invalid value should give a message that names the parameter and the expected value, not a stack trace or an empty result.
style="foo" and source="foo" give "Can't find descriptor for the component with type [interface org.xwiki.contrib.jira.macro.JIRADisplayer] and hint [foo]", with no list of the allowed values.
url="notaurl" gives "no protocol: notaurl".
parameters="garbage" gives "Invalid macro parameters used for the [jira] macro. Cause: [null]".
maxCount="abc" and maxCount="0" render an empty table with only its headers, and fields=":::,," renders nothing, with no message at all.
The messages for an unknown id and for a wrong issue key or JQL query are already clear. So the idea would be a single place that turns a failure into a short, translatable message for the reader, whether the cause is the server being unavailable or a wrong parameter value, with the technical details kept in the collapsible part.
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