Allow update server administrator to return a message to the Joomla admin.

Pull Request for Issue # .

Summary of Changes

Add a preupdatemessage field to the XML returned by the update server.

Testing Instructions

Install test plugin:
https://www.brainforge.co.uk/media/extra_downloads/test/test.zip

Check for updates and the test plugin will appear.
Tick the box and run update.
In this example a simple error message will appear and the update aborted.
... a warning / information message could been used instead.

Actual result BEFORE applying this Pull Request

Update works as expected - although the version number is unchanged!
So it only temporarily disappears from the available update list.

Expected result AFTER applying this Pull Request

Error message displayed and the update does not happen.

The benefit of this is that an update server manager can send a message to the Joomla administrator.
e.g. 'Please contact us.'

The following documentation requires updating:
https://manual.joomla.org/docs/building-extensions/install-update/update-server/
See file: https://www.brainforge.co.uk/media/extra_downloads/test/updatetest.xml

Link to documentations

Please select:

• Documentation link for docs.joomla.org:

• No documentation changes for docs.joomla.org needed

• Pull Request link for manual.joomla.org:

• No documentation changes for manual.joomla.org needed