fix s3, not all storages have "path" attribute

When the current storage was a type that did not have a path attribute (S3BotoStorage from django_storage, for example) the bare except was swallowing the error and just returning <empty>, even though it does have an image. Image.name comes from Django so seems more reliable.

Not really sure what the except is supporting, but AttributeError seems the only conceivable thing.

This is the error is caused when I tried to upload an image with `DEFAULT_FILE_STORAGE = 'storages.backends.s3boto.S3BotoStorage'`

```
 File "/app/.heroku/python/lib/python2.7/site-packages/django/core/handlers/base.py", line 112, in get_response
   response = wrapped_callback(request, *callback_args, **callback_kwargs)

 File "/app/.heroku/python/lib/python2.7/site-packages/django/utils/decorators.py", line 99, in _wrapped_view
   response = view_func(request, *args, **kwargs)

 File "/app/.heroku/python/lib/python2.7/site-packages/django/views/decorators/cache.py", line 52, in _wrapped_view_func
   response = view_func(request, *args, **kwargs)

 File "/app/.heroku/python/lib/python2.7/site-packages/django/contrib/admin/sites.py", line 198, in inner
   return view(request, *args, **kwargs)

 File "/app/.heroku/python/lib/python2.7/site-packages/cms/admin/pageadmin.py", line 1485, in edit_plugin
   return super(PageAdmin, self).edit_plugin(*args, **kwargs)

 File "/app/.heroku/python/lib/python2.7/site-packages/django/views/decorators/clickjacking.py", line 41, in wrapped_view
   resp = view_func(*args, **kwargs)

 File "/app/.heroku/python/lib/python2.7/site-packages/cms/admin/placeholderadmin.py", line 389, in edit_plugin
   response = plugin_admin.add_view(request)

 File "/app/.heroku/python/lib/python2.7/site-packages/django/utils/decorators.py", line 29, in _wrapper
   return bound_func(*args, **kwargs)

 File "/app/.heroku/python/lib/python2.7/site-packages/django/utils/decorators.py", line 99, in _wrapped_view
   response = view_func(request, *args, **kwargs)

 File "/app/.heroku/python/lib/python2.7/site-packages/django/utils/decorators.py", line 25, in bound_func
   return func(self, *args2, **kwargs2)

 File "/app/.heroku/python/lib/python2.7/site-packages/django/db/transaction.py", line 371, in inner
   return func(*args, **kwargs)

 File "/app/.heroku/python/lib/python2.7/site-packages/django/contrib/admin/options.py", line 1134, in add_view
   return self.response_add(request, new_object)

 File "/app/.heroku/python/lib/python2.7/site-packages/cms/plugin_base.py", line 221, in response_add
   return super(CMSPluginBase, self).response_add(request, obj, **kwargs)

 File "/app/.heroku/python/lib/python2.7/site-packages/django/contrib/admin/options.py", line 930, in response_add
   'obj': escapejs(obj)

 File "/app/.heroku/python/lib/python2.7/site-packages/django/utils/functional.py", line 203, in wrapper
   return func(*args, **kwargs)

 File "/app/.heroku/python/lib/python2.7/site-packages/django/utils/html.py", line 61, in escapejs
   return mark_safe(force_text(value).translate(_js_escapes))

TypeError: expected a character buffer object


```