broken shelf file causes unloadable plugin

[drevicko]

as far as I can see, if your plugin fails to pickle it's shelf properly (eg: this issue), it then becomes impossible to load the plugin, as an EOF exception is thrown similar to this:

ERROR:senpy.extensions:Error activating plugin emotion-wnaffect -  : 
    Traceback (most recent call last):
  File "/Users/drevicko/anaconda/envs/python2/lib/python2.7/site-packages/senpy/extensions.py", line 290, in act
    plugin.activate()
  File "./emotion-wnaffect.py", line 73, in activate
    if 'total_synsets' not in self.sh:
  File "/Users/drevicko/anaconda/envs/python2/lib/python2.7/site-packages/senpy/plugins/__init__.py", line 99, in sh
    self.__dict__['_sh'] = pickle.load(open(self.shelf_file, 'rb'))
  File "/Users/drevicko/anaconda/envs/python2/lib/python2.7/pickle.py", line 1384, in load
    return Unpickler(file).load()
  File "/Users/drevicko/anaconda/envs/python2/lib/python2.7/pickle.py", line 864, in load
    dispatch[key](self)
  File "/Users/drevicko/anaconda/envs/python2/lib/python2.7/pickle.py", line 886, in load_eof
    raise EOFError
EOFError

It would seem a good idea to issue a warning and fall back on starting the plugin from scratch.

[balkian]

Thanks for the contribution, but I think it's better to throw an exception (and avoid removing the file) unless explicitly told not to.

I've made this change to keep backwards compatibility. It also covers other unpickling errors, not just empty files.

You can now add force_shelf to your plugin definitions.