There is a recurrent ID when the city requested doesn't exist, you can maybe based your code on that, or make a second request. I've explained the two solutions that I would have used.
#!/usr/bin/python
import urllib2, json
city = "etre4t5r5e4re"
root = "http://openweathermap.org/data/2.1/forecast/city?q=%s"
url = root % city
response = urllib2.urlopen(url)
j = json.load(response)
# Solution 1
if j.get('url', '').split('/')[-1] == '7284885':
print " ! This city seems to be THE Unknown city"
# Solution 2
if 'No station' in urllib2.urlopen(j.get('url')).read():
print " ! Again.. This city seems to be THE Unknown city"