Top level services cannot silence errors from lower level services

Whenever a service is configured to use another service for routing (i.e. it uses targets), the errors that are logged whenever the connection creation fails are not suppressed even if the upper level service is capable of dealing with them gracefully.

This requires a chage in the routing API to return an error object of some sorts to the caller like is done in 23.08 with the handleError function where the errors are propagated upwards until something handles them or they reach the MXS_SESSION which then terminates the connection.