change validation of WP children

- do not add error on a symbol called :#123 (123 is the childrens ID)
    - this is against the semantics of ActiveRecord::Errors
    - creating symbols in a loop used to be a bad idea (might be better today)
    - it breaks error messages in the APIv3 (which is due to APIv3 working beyond the borders of AR errors, not the best of ideas...)
- use :base instead, the place for unlocatable errors
- include the fact that #123 refers to a child element in the error message

Basically I feel that this makes the children validation cleaner and better. But somehow I fear, that there is some code out there that depends on the fact that error keys like :#123 existed...

app/models/work_package/validations.rb

@@ -125,8 +125,9 @@ module WorkPackage::Validations
 
   def validate_children
     children.select { |c| !c.valid? }.each do |child|
-      child.errors.each do |_, value|
+      child.errors.each do |attribute, message|
-        errors.add(:"##{child.id}", value)
+        full_message = child.errors.full_message(attribute, message)
+        errors.add(:base, "#{I18n.t('label_child_element')} ##{child.id}: #{full_message}")
       end
     end
   end

config/locales/en.yml

@@ -674,6 +674,7 @@ en:
   label_changes_details: "Details of all changes"
   label_changeset_plural: "Changesets"
   label_checked: "checked"
+  label_child_element: "Child element"
   label_chronological_order: "In chronological order"
   label_close_versions: "Close completed versions"
   label_closed_work_packages: "closed"

spec/requests/api/v3/work_package_resource_spec.rb

@@ -897,7 +897,9 @@ h4. things we like
 
           it_behaves_like 'multiple errors of the same type with messages' do
             let(:message) {
-              [child_1.id, child_2.id].map { |id| "##{id} cannot be in another project." }
+              [child_1.id, child_2.id].map { |id|
+                "Child element ##{id}: Parent cannot be in another project."
+              }
             }
           end
         end