Run `Ci::Runner#assign_to` in transaction

app/models/ci/runner.rb

@@ -120,8 +120,15 @@ module Ci
         raise ArgumentError, 'Transitioning a group runner to a project runner is not supported'
       end
 
+      begin
+        transaction do
           self.projects << project
-      self.save
+          self.save!
+        end
+      rescue ActiveRecord::RecordInvalid => e
+        self.errors.add(:assign_to, e.message)
+        false
+      end
     end
 
     def display_name