Symfony version(s) affected: 3.4.11

Description
There is a small bug in an exception thrown by RegisterEventListenersAndSubscribersPass when a Doctrine connection that doesn't exist is referenced in configuration. Instead of getting the helpful exception message:

The Doctrine connection "root" referenced in service "stof_doctrine_extensions.listener.sluggable" does not exist. Available connections names: default

You instead get

Notice: Array to string conversion

How to reproduce
I was able to reproduce this by having the following configuration:

stof_doctrine_extensions:
    orm:
        root:
            sluggable: true
            timestampable: true

after I removed the connection named root.

Possible Solution
The variable $taggedSubscriber here is actually an array, with the name of the service as the first member. Changing the reference to $taggedSubscriber[0] should fix this.

https://github.com/symfony/symfony/blob/master/src/Symfony/Bridge/Doctrine/DependencyInjection/CompilerPass/RegisterEventListenersAndSubscribersPass.php#L72

Additional context