Previous issues have introduced code that stops an upgrade while it is in process when an old plugin is detected. It is good that the code is detecting invalid plugins, but stopping an upgrade part-way through is too late and rather disappointing.

It would be good to detect such plugins before the DB upgrade commences and prevent the upgrade from starting until the plugin is upgraded for the target version.

Replications steps:

1. Have a 1.9 site with some contributed plugins
2. Upgrade the Moodle code, but not the plugin code
3. The new Moodle code should be detected and the admin will be prompted to start the upgrade process
4. The Plugins check page is displayed. Take note of what is shown for the contributed plugins.
5. At the bottom of the page, click Upgrade. The DB upgrade will commence.

Expected result: Old plugins should haven been noted in the Plugins check. The DB upgrade should not have been able to start until new versions of contributed plugins are used or old plugins are removed.

Actual result: The DB ugprade stops with an error when it reaches a 1.9 plugin.