Fatal message should terminate XSL

[robander]

Description

Updates the output-message routine so that if a message is defined with type="FATAL", the XSL will terminate.

Motivation and Context

I was trying to add a fatal-severity message to a custom XSL file, and noticed that it was generated (with [FATAL] in the message) but that the build continued to the normal end. A few years back, this would have stopped the build

Cause: messages used to be generated by passing in a message ID, a message, and a severity, which were combined with a hard-coded prefix. If the severity was F, the message was generated in a way that terminated the process.

This method of calling messages was deprecated some releases ago; now you just pass in the ID and optional parameters. When a message is fatal, the message definition should specify type="FATAL". We do not ship any fatal messages intended for XSL but it should be possible to create them; here's the syntax of another fatal message:
<message id="DOTA001F" type="FATAL">

This code update fixes the code so that in addition to checking the old param for $msgsev='F', it reads the message and checks for @type='FATAL'.

How Has This Been Tested?

1. Created a section with two <title> elements
2. Verify it generates the message DOTX041W
3. Change the message definition to fatal: <message id="DOTX041W" type="FATAL">
4. Rebuild, and processing of that topic now quits with:

     [xslt] file:/C:/Users/IBM_ADMIN/github/robander/dita-ot/src/main/docsrc/samples/concepts/lawnmower.xml:14:10: [DOTX041W][FATAL]: Found more than one title element in a section element. Using the first one for the section's title.
     [xslt] Failed to transform document: Processing terminated by xsl:message at line 99 in output-message.xsl

Type of Changes

Bug fix - restores a simple feature that was broken / unavailable unless using deprecated code.