Don’t 500 when a CSV is missing rows

> When the CSV is missing the header row, we get an error and the user
> will see "Sorry, we are experiencing technical difficulties..."
>
> We should return a better error message for the user.

– https://www.pivotaltracker.com/story/show/140668615

This was caused by an attempt to access the `first_recipient` variable
before it was assigned. It would only be assigned when there was at
least one row in the file.

Fixing this means doing two things:
- defaulting `first_recipient` to be `None` before looking in the file
- adding an error message for when we can’t extract any rows out of the
  file (which is more nuanced than the file just being completely empty)

(There’s a nasty `sort` in the Jinja template because when there are no
rows in the file the order of the required column headers is not
deterministic.)

tests/app/main/views/test_send.py

@@ -138,7 +138,23 @@ def test_upload_csvfile_with_errors_shows_check_page_with_errors(
             'It doesn’t have a column called ‘name’. '
             'Skip to file contents'
         )
-    )
+    ),
+    (
+        "+447700900986",
+        (
+            'Your file is missing some rows '
+            'It needs at least one row of data, and columns called ‘name’ and ‘phone number’. '
+            'Skip to file contents'
+        )
+    ),
+    (
+        "",
+        (
+            'Your file is missing some rows '
+            'It needs at least one row of data, and columns called ‘name’ and ‘phone number’. '
+            'Skip to file contents'
+        )
+    ),
 ])
 def test_upload_csvfile_with_missing_columns_shows_error(
     logged_in_client,