run.py doesn't handle properly missing BACKUP_DEFAULT_DEST

```
→ python run.py --backup
('secrets', '/Users/karl/code/webcompat.com')
Traceback (most recent call last):
  File "run.py", line 125, in <module>
    if not os.path.exists(app.config['BACKUP_DEFAULT_DEST']):
KeyError: 'BACKUP_DEFAULT_DEST'
```

Also there is a big cleanup to do for the syntax, lines too long, etc.
