Fix error when uploading malformed CSV import (#19509)

app/validators/import_validator.rb

@@ -26,6 +26,8 @@ class ImportValidator < ActiveModel::Validator
     when 'following'
       validate_following_import(import, row_count)
     end
+  rescue CSV::MalformedCSVError
+    import.errors.add(:data, :malformed)
   end
 
   private

config/locales/activerecord.en.yml

@@ -29,6 +29,10 @@ en:
           attributes:
             website:
               invalid: is not a valid URL
+        import:
+          attributes:
+            data:
+              malformed: is malformed
         status:
           attributes:
             reblog: