Add check before removing old notes column

Check for content in PublicBody::Translation#notes column.

If a re-user skips 0.42.0.0 upgrade notes then they could lose data. Add
a check to ensure we prevent this.

db/migrate/20220902112339_remove_public_body_notes.rb

@@ -1,13 +1,22 @@
 class RemovePublicBodyNotes < ActiveRecord::Migration[6.1]
-  def change
-    reversible do |dir|
-      dir.up do
-        remove_column :public_body_translations, :notes
-      end
-
-      dir.down do
-        PublicBody.add_translation_fields! notes: :text
-      end
+  def up
+    if PublicBody::Translation.where.not(notes: nil).any?
+      raise <<~TXT
+        We can't run the RemovePublicBodyNotes database migrations.
+
+        We have dectected PublicBody::Translation objects which haven't been
+        migrated to the new Note model.
+
+        Please deploy Alaveteli 0.42.0.0 and run the upgrade tasks:
+        https://github.com/mysociety/alaveteli/blob/0.42.0.0/doc/CHANGES.md#upgrade-notes
+
+      TXT
     end
+
+    remove_column :public_body_translations, :notes
+  end
+
+  def down
+    PublicBody.add_translation_fields! notes: :text
   end
 end