Catch ZLIB error

[atoponce]

When decrypting the note, it is possible that the recipient inputs the passphrase incorrectly, thus throwing the following ZLIB error. The recipient should be presented with providing the passphrase again, making infinite attempts at decrypting the note.

  File "/usr/share/pyshared/flask/app.py", line 1836, in __call__
    return self.wsgi_app(environ, start_response)
  File "/usr/share/pyshared/flask/app.py", line 1820, in wsgi_app
    response = self.make_response(self.handle_exception(e))
  File "/usr/share/pyshared/flask/app.py", line 1403, in handle_exception
    reraise(exc_type, exc_value, tb)
  File "/usr/share/pyshared/flask/app.py", line 1817, in wsgi_app
    response = self.full_dispatch_request()
  File "/usr/share/pyshared/flask/app.py", line 1477, in full_dispatch_request
    rv = self.handle_user_exception(e)
  File "/usr/share/pyshared/flask/app.py", line 1381, in handle_user_exception
    reraise(exc_type, exc_value, tb)
  File "/usr/share/pyshared/flask/app.py", line 1475, in full_dispatch_request
    rv = self.dispatch_request()
  File "/usr/share/pyshared/flask/app.py", line 1461, in dispatch_request
    return self.view_functions[rule.endpoint](**req.view_args)
  File "/home/aaron/src/d-note/d-note.py", line 145, in fetch_url
    plaintext = note_decrypt(privkey, random_url)
  File "/home/aaron/src/d-note/d-note.py", line 90, in note_decrypt
    return zlib.decompress(unpad(plaintext))
error: Error -3 while decompressing data: unknown compression method