add - catch page errors when 'fullurl' is missing.

When requesting some pages via WikipediaPage():

````
  File "/usr/local/lib/python2.7/dist-packages/wikipedia/wikipedia.py", line 224, in __init__
    self.load(redirect=redirect, preload=preload)
  File "/usr/local/lib/python2.7/dist-packages/wikipedia/wikipedia.py", line 276, in load
    self.__init__(title, redirect=redirect, preload=preload)
  File "/usr/local/lib/python2.7/dist-packages/wikipedia/wikipedia.py", line 224, in __init__
    self.load(redirect=redirect, preload=preload)
  File "/usr/local/lib/python2.7/dist-packages/wikipedia/wikipedia.py", line 296, in load
    self.url = data['fullurl']
KeyError: 'fullurl'
````

I think it may be due to case & whitespace in the article title string?  In any case, raising a PageError in this case may be more appropriate to pass back to the client.