Show an error message if fetching fails

When I first tried to scrape, I got an error that's not very informative:
```
Traceback (most recent call last):
  File "Scraper.py", line 69, in <module>
    for i in range(config.start_entry, nt.last_entry + 1):
  File "/home/flan/.src/nyaamagnet/Nyaa.py", line 14, in last_entry
    link = soup.find('tr', class_='tlistrow').find('td', class_='tlistname').a['href']
AttributeError: 'NoneType' object has no attribute 'find'
```