make partials source-agnostic

they're currently expecting a RecipientsCSV object - but we won't
always have that available if we're handling a single notification.

So make the partials take generic variables, and then use a jinja with
block to pass in the correct values from either the check csv page or
the check notification page.

Additionally set the notification check page to show errors nicely -
hide the send button if there were problems, and replace the header

app/templates/partials/check/message-too-long.html

@@ -4,7 +4,8 @@
       Message too long
     </h1>
     <p>
-      Text messages can’t be longer than {{ template|string|length }} characters. Your message is {{message_length}} characters.
+      Text messages can’t be longer than {{ SMS_CHAR_COUNT_LIMIT }} characters.
+      Your message is {{ template.content_count }} characters.
     </p>
   {% endcall %}
 </div>

app/templates/partials/check/not-allowed-to-send-to.html

@@ -1,11 +1,19 @@
+{% from "components/banner.html" import banner_wrapper %}
+
+{% macro skip_to_file_contents() %}
+  <p class="visually-hidden">
+    <a href="#{{ file_contents_header_id }}">Skip to file contents</a>
+  </p>
+{% endmacro %}
+
 <div class="bottom-gutter">
   {% call banner_wrapper(type='dangerous') %}
     <h1 class='banner-title'>
       You can’t send to
       {{ 'this' if count_of_recipients == 1 else 'these' }}
-      {{ recipients.recipient_column_headers[0] }}
+      {{ template_type_label }}
       {%- if count_of_recipients != 1 -%}
-        {{ 'es' if 'email address' == recipients.recipient_column_headers[0] else 's' }}
+        {{ 'es' if 'email address' == template_type_label else 's' }}
       {%- endif %}
     </h1>
     <p>