fix(services): service instance create/update errors (cloudfoundry#1736)

- Fixes an issue where updating a user-provided service instance to have
the same name as an existing service instance caused a HTTP 500 Internal
Server Error
- Makes message validation errors more consistent with other endpoints -
they now fail with HTTP 422 Unprocessable Entity
- Improves testing of error details

[#173416145](https://www.pivotaltracker.com/story/show/173416145)

Author: blgm

app/actions/service_instance_update_user_provided.rb

@@ -4,7 +4,7 @@ module VCAP::CloudController
   class ServiceInstanceUpdateUserProvided
     include ServiceInstanceCreateMixin
 
-    class InvalidUserProvidedServiceInstance < ::StandardError
+    class UnprocessableUpdate < ::StandardError
     end
 
     def initialize(service_event_repository)
@@ -37,7 +37,7 @@ def update(service_instance, message)
     attr_reader :service_event_repository
 
     def error!(message)
-      raise InvalidUserProvidedServiceInstance.new(message)
+      raise UnprocessableUpdate.new(message)
     end
   end
 end

app/controllers/v3/service_instances_controller.rb

@@ -229,16 +229,18 @@ def create_managed(message, space:)
 
   def update_user_provided(service_instance)
     message = ServiceInstanceUpdateUserProvidedMessage.new(hashed_params[:body])
-    bad_request!(message.errors.full_messages) unless message.valid?
+    unprocessable!(message.errors.full_messages) unless message.valid?
 
     service_event_repository = VCAP::CloudController::Repositories::ServiceEventRepository::WithUserActor.new(user_audit_info)
     service_instance = ServiceInstanceUpdateUserProvided.new(service_event_repository).update(service_instance, message)
     render status: :ok, json: Presenters::V3::ServiceInstancePresenter.new(service_instance)
+  rescue ServiceInstanceUpdateUserProvided::UnprocessableUpdate => api_err
+    unprocessable!(api_err.message)
   end
 
   def update_managed(service_instance)
     message = ServiceInstanceUpdateManagedMessage.new(hashed_params[:body])
-    bad_request!(message.errors.full_messages) unless message.valid?
+    unprocessable!(message.errors.full_messages) unless message.valid?
 
     if message.service_plan_guid
       service_plan = ServicePlan.first(guid: message.service_plan_guid)
@@ -315,15 +317,15 @@ def can_write_space?(space)
 
   def build_create_message(params)
     generic_message = ServiceInstanceCreateMessage.new(params)
-    invalid_param!(generic_message.errors.full_messages) unless generic_message.valid?
+    unprocessable!(generic_message.errors.full_messages) unless generic_message.valid?
 
     specific_message = if generic_message.type == 'managed'
                          ServiceInstanceCreateManagedMessage.new(params)
                        else
                          ServiceInstanceCreateUserProvidedMessage.new(params)
                        end
 
-    invalid_param!(specific_message.errors.full_messages) unless specific_message.valid?
+    unprocessable!(specific_message.errors.full_messages) unless specific_message.valid?
     specific_message
   end