Symfony version(s) affected: 4.3

Description
At least in 4.3, if you have multiple messenger buses, you need to specify framework.messenger.default_bus. If you do not, you'll get:

The service "messenger.routable_message_bus" has a dependency on a non-existent service "messenger.default_bus"

How to reproduce
Add framework.messenger.buses with two entries below, but do not specify framework.messenger.default_bus.

Possible Solution
In the FrameworkBundle Configuration class, we should probably add validation to throw a better message.