Improve and clarify large task error handling

Previously we changed this as an experiment [1] and forgot to check
if the new exception was appropriate. It is: testing with a large
task invocation ('process_job.apply_async(["a" * 200000])') gives...

Before (Celery 3, large-ish task):

    boto.exception.SQSError: SQSError: 400 Bad Request
    <?xml version="1.0"?><ErrorResponse xmlns="http://queue.amazonaws.com/doc/2012-11-05/"><Error><Type>Sender</Type><Code>InvalidParameterValue</Code><Message>One or more parameters are invalid. Reason: Message must be shorter than 262144 bytes.</Message><Detail/></Error><RequestId>96162552-cd96-5a14-b3a5-7f503300a662</RequestId></ErrorResponse>

Before (Celery 3, very large task):

    <hangs forever>

After (Celery 5, large-ish task):

    botocore.exceptions.ClientError: An error occurred (InvalidParameterValue) when calling the SendMessage operation: One or more parameters are invalid. Reason: Message must be shorter than 262144 bytes.

After (Celery 5, very large task):

    botocore.parsers.ResponseParserError: Unable to parse response (syntax error: line 1, column 0), invalid XML received. Further retries may succeed:
    b'HTTP content length exceeded 1662976 bytes.'

app/v2/notifications/post_notifications.py

@@ -232,9 +232,10 @@ def process_sms_or_email_notification(
                 reply_to_text=reply_to_text
             )
             return resp
-        except botocore.exceptions.ClientError:
-            # if SQS cannot put the task on the queue, it's probably because the notification body was too long and it
-            # went over SQS's 256kb message limit. If so, we
+        except (botocore.exceptions.ClientError, botocore.parsers.ResponseParserError):
+            # If SQS cannot put the task on the queue, it's probably because the notification body was too long and it
+            # went over SQS's 256kb message limit. If the body is very large, it may exceed the HTTP max content length;
+            # the exception we get here isn't handled correctly by botocore - we get a ResponseParserError instead.
             current_app.logger.info(
                 f'Notification {notification_id} failed to save to high volume queue. Using normal flow instead'
             )