diagnostic message of failed `cf update-service-broker` is unfriendly

Issue

diagnostic message of failed cf update-service-broker is unfriendly in the following scenario

• given a service broker has registered a service with name overview-service and guid G1
• given an existing service instance for service overview-service
• given a broker updates changes response from its v2/catalog to return a service with name overview-service and guid G2
• when admin executes cf update-service-broker
• then the admin sees

Server error, status code: 502, error code: 270012, message: Service broker catalog is invalid: 
Service names must be unique within a broker. Services with names ["overview-service"] already exist

The message is unfriendly in the sense that:

• the catalog per se is valid
• however, given existing provisionned service instances, the service offering with guid G1 can not be unloaded and thus conflicts with offering with guid G2

Context

As a CF admin

• in order to be productive with CF API
• I need helpful friendly diagnostic messages on errors

Steps to Reproduce

• deploy broker from https://github.com/cloudfoundry/overview-broker
• register the broker and enable its service plan
• provision a service instance cf create-service overview-service small
• restart the broker, this results into catalog to be regenerated with different guid, see https://github.com/cloudfoundry/overview-broker/issues/71
• cf update-service-broker

Expected result

Server error, status code: 502, error code: 270012, message: Service broker catalog can not be updated: 
Service names must be unique within a broker. Service with name "overview-service" and guid "a49cb559-5539-447b-9fdd-bdaa3970606d" already exists with guid "cf40e289-d0ac-4f80-a82c-36dbad79ede1" and can not be unloaded as there are existing service instances attached.

Current result

Server error, status code: 502, error code: 270012, message: Service broker catalog is invalid: 
Service names must be unique within a broker. Services with names ["overview-service"] already exist

Start at the cf update-service-broker flow and its v2/catalog handling, then trace where error code 270012 turns the duplicate-name conflict into the current diagnostic. Reproduce the overview-broker sequence and make the failure report the catalog update, both service GUIDs, and the existing service-instance constraint as shown in the expected result.