Misleading confirmation message when activating network-only plugins

Plugins can mark themselves as being network-only by including the following header:

`Network: true`

If you attempt to activate such a plugin with WP-CLI then the plugin will be activated (network-wide) but the confirmation will say otherwise.

To reproduce:

`wp plugin install background-update-tester --activate`

WP-CLI will report:

    Plugin installed successfully.
    Activating 'background-update-tester'...
    Warning: Could not activate the 'background-update-tester' plugin.