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>