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.)

app/main/views/send.py

@@ -249,8 +249,6 @@ def _check_messages(service_id, template_type, upload_id, letters_as_pdf=False):
     remaining_messages = (current_service['message_limit'] - sum(stat['requested'] for stat in statistics.values()))
 
     contents = s3download(service_id, upload_id)
-    if not contents:
-        flash('There was a problem reading your upload file')
 
     template = get_template(
         service_api_client.get_service_template(
@@ -295,6 +293,7 @@ def _check_messages(service_id, template_type, upload_id, letters_as_pdf=False):
         choose_time_form = ChooseTimeForm()
 
     with suppress(StopIteration):
+        first_recipient = None
         template.values = next(recipients.rows)
         first_recipient = template.values.get(
             Columns.make_key(recipients.recipient_column_headers[0]),