tests: style fix

Signed-off-by: Rafał Malinowski <rafal.przemyslaw.malinowski@gmail.com>
plugins: deactivate plugins with dependents

Signed-off-by: Rafał Malinowski <rafal.przemyslaw.malinowski@gmail.com>
plugins: set PluginStateDisabled from outside of plugin code

Signed-off-by: Rafał Malinowski <rafal.przemyslaw.malinowski@gmail.com>
plugins: set PluginStateEnabled/PluginStateDisabled from outside of plugin code

Signed-off-by: Rafał Malinowski <rafal.przemyslaw.malinowski@gmail.com>
plugins: use exception to inform about activation failure

This is still not pretty code, but it will be ;)

Signed-off-by: Rafał Malinowski <rafal.przemyslaw.malinowski@gmail.com>