Fix error message when loading Py3 model in Py2

[JaimieMurdock]

Add a catch for this error which will occur when Python 3 models are attempted to be loaded into Python 2.

pujianguaideAir:Desktop pjhuai$ topicexplorer launch /Users/pjhuai/Desktop/Analects-20.ini 
using default label function
using default id function
Traceback (most recent call last):
  File "/Users/pjhuai/anaconda/bin/topicexplorer", line 11, in <module>
    load_entry_point('topicexplorer', 'console_scripts', 'topicexplorer')()
  File "/Users/pjhuai/inpho/topicexplorer/topicexplorer/__main__.py", line 242, in main
    app = benchmark(server.create_app)(args)
  File "/Users/pjhuai/inpho/topicexplorer/topicexplorer/server.py", line 674, in create_app
    home_link=home_link)
  File "/Users/pjhuai/inpho/topicexplorer/topicexplorer/server.py", line 89, in __init__
    self._load_corpus(corpus_file)
  File "/Users/pjhuai/inpho/topicexplorer/topicexplorer/server.py", line 120, in _load_corpus
    self.c = Corpus.load(corpus_file)
  File "/Users/pjhuai/anaconda/lib/python2.7/site-packages/vsm/corpus/base.py", line 623, in load
    return Corpus._parallel_load(file)
  File "/Users/pjhuai/anaconda/lib/python2.7/site-packages/vsm/zipfile.py", line 29, in wrapper
    value = f(*args, **kwargs)
  File "/Users/pjhuai/anaconda/lib/python2.7/site-packages/vsm/corpus/base.py", line 664, in _parallel_load
    c.context_data = [future.result() for future in c.context_data]
  File "/Users/pjhuai/anaconda/lib/python2.7/site-packages/concurrent/futures/_base.py", line 422, in result
    return self.__get_result()
  File "/Users/pjhuai/anaconda/lib/python2.7/site-packages/concurrent/futures/thread.py", line 62, in run
    result = self.fn(*self.args, **self.kwargs)
  File "/Users/pjhuai/anaconda/lib/python2.7/site-packages/vsm/corpus/base.py", line 35, in load_npz
    return zipfile.__getitem__(obj)
  File "/Users/pjhuai/anaconda/lib/python2.7/site-packages/numpy/lib/npyio.py", line 233, in __getitem__
    pickle_kwargs=self.pickle_kwargs)
  File "/Users/pjhuai/anaconda/lib/python2.7/site-packages/numpy/lib/format.py", line 640, in read_array
    array = pickle.load(fp, **pickle_kwargs)
ValueError: non-string names in Numpy dtype unpickling