Harmonise content of error message with the document laid out by our content designer

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

@@ -1,6 +1,8 @@
 <h1 class='banner-title' data-module="track-error" data-error-type="Trying to send a letter that's too long" data-error-label="{{ upload_id }}">
-  This letter is too long
+  Your letter is too long
 </h1>
 <p>
-  Letters must be {{ letter_max_pages }} pages or fewer
+  Letters must be {{ letter_max_pages }} pages or less.
+  <br>
+  Your letter is {{ page_count }} pages long.
 </p>