Is there any way of getting an error message from the browsers XslProcessor object when using xsl:message terminate=true?

Here is what I am using:

<?xml version="1.0" encoding="utf-8"?>
<songList/>

xsl:

<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
<xsl:output method="xml" omit-xml-declaration="yes"/>
<xsl:template match="/">
    <out>ROOT found</out>   
    <xsl:message terminate="yes"><xsl:value-of select="'Test message - ZZZ'"/></xsl:message>
</xsl:template>
</xsl:stylesheet>

I have an html page that calls this and when the terminate=”no” (or I remove this line completely) the result is (as expected);

<out>ROOT found</out> 

The code is pretty much what is to be found in say MDN, eg

try{
    resultDoc = processor.transformToFragment(xmlDoc, document);
} catch (err) {
    console.log('err=' + err);
    return false;
}   

My issue is that whilst I am using FF and gecko, and the page here
https://developer.mozilla.org/en-US/docs/Web/XSLT/Element/message#gecko_support
seems to indicate xsl:message is supported, it only appears to be supported in the very loosest sense, ie it halts processing but it does not pass back the error/informational message so is of limited utility.
Am I doing something incorrectly?
When I set a breakpoint on the
console.log(‘err=’ + err); line it is totally unpopulated.
Yes, it does get hit but any error information is simply not present.