This set of changes attempts to handle missing plugins in a better fashion.
Rather than throwing an uncaught error which causes a unrecoverable site
error, which in turn means that no user can use any other plugin; this
patch disables the problem plugin and informs administrators of the issue.

To handle the case where core plugins, which under normal circumstances
cannot be disabled, are missing and disabled; the plugins.tpl template is
also changed to test for the contents of the activateform, instead of
whether the plugin can ordinarily be disabled. This means that if the issue
is then fixed, the plugin can once again be re-enabled, at which point the
[ hide ] link will no longer appear.

Change-Id: I6f9fa73ae0ac307b5f5f8f30b8205690926f54fc
Signed-off-by: Andrew Robert Nicols <andrew.nicols@luns.net.uk>