Fix error message when recipient column missing

When your CSV file is missing the recipient column (eg ‘phone number’
or ‘email address’) we give you a helpful error message telling you that
this is the case.

When we changed the recipient column to be columns, plural, we didn’t
update the code that generated the error message. So you would get
errors that looked this like this:

> Your file needs to have a column called ‘’

This commit fixes the error message.

app/templates/views/check.html

@@ -41,7 +41,11 @@
     <div class="bottom-gutter">
       {% call banner_wrapper(type='dangerous') %}
         <h1 class='banner-title'>
-          Your file needs to have a column called ‘{{ recipients.recipient_column_header }}’
+          Your file needs to have {{ formatted_list(
+            recipients.recipient_column_headers,
+            prefix='a column called',
+            prefix_plural='columns called'
+          ) }}
         </h1>
         <p>
           Your file has {{ formatted_list(