Bug 19081: Do not list plugins that have been uninstalled

Bug 19081: Do not list plugins that have been uninstalled

Under plack, can_load should not check if a package is in cache, but
reload it. Otherwise plugins that have been uninstalled will still get
listed.
The error raised by can_load must only be displayed if the plugin has
been removed.

Test plan:
1/ Upload a plugin
2/ Note the plugin is listed as installed
3/ Modify the package of the plugin to add a compilation error (use
'Foo' for instance)
4/ Reload the page
5/ The plugin is not listed and a warning appear in the logs
6/ Remove the compilation error and uninstall the plugin
7/ The plugin is no longer listed and no warning appear in the logs

Signed-off-by: Lee Jamison <ldjamison@marywood.edu>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Bug 19081: Remove useless $plugin_file variable

Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>