Update project import API field

lib/api/entities/project_import_failed_relation.rb

@@ -3,7 +3,11 @@
 module API
   module Entities
     class ProjectImportFailedRelation < Grape::Entity
-      expose :id, :created_at, :exception_class, :exception_message, :source
+      expose :id, :created_at, :exception_class, :source
+
+      expose :exception_message do |_|
+        nil
+      end
 
       expose :relation_key, as: :relation_name
     end

spec/lib/api/entities/project_import_failed_relation_spec.rb

@@ -14,7 +14,7 @@ RSpec.describe API::Entities::ProjectImportFailedRelation do
         id: import_failure.id,
         created_at: import_failure.created_at,
         exception_class: import_failure.exception_class,
-        exception_message: import_failure.exception_message,
+        exception_message: nil,
         relation_name: import_failure.relation_key,
         source: import_failure.source
       )