Made ImageLoader.zip_loader skip loaders that raised an error or returned None.

For some reason one of the image loaders was failing on Mac OS X. Running the widgets/sequence_images example:

    [DEBUG  ] [ImageImageIO] Load <buttonoverlay_dn_001.png> from <data/images/button_white_animated.zip>
    [WARNING] [Image       ] Unable to load image<buttonoverlay_dn_001.png> in zip <data/images/button_white_animated.zip> trying to continue...
    [DEBUG  ] [ImageImageIO] Load <buttonoverlay_dn_002.png> from <data/images/button_white_animated.zip>
    [WARNING] [Image       ] Unable to load image<buttonoverlay_dn_002.png> in zip <data/images/button_white_animated.zip> trying to continue...
    [DEBUG  ] [ImageImageIO] Load <buttonoverlay_dn_003.png> from <data/images/button_white_animated.zip>
    [WARNING] [Image       ] Unable to load image<buttonoverlay_dn_003.png> in zip <data/images/button_white_animated.zip> trying to continue...
    [DEBUG  ] [ImageImageIO] Load <buttonoverlay_dn_004.png> from <data/images/button_white_animated.zip>
    [WARNING] [Image       ] Unable to load image<buttonoverlay_dn_004.png> in zip <data/images/button_white_animated.zip> trying to continue...
    [DEBUG  ] [ImageImageIO] Load <buttonoverlay_dn_005.png> from <data/images/button_white_animated.zip>
    [WARNING] [Image       ] Unable to load image<buttonoverlay_dn_005.png> in zip <data/images/button_white_animated.zip> trying to continue...
    [DEBUG  ] [ImageImageIO] Load <buttonoverlay_dn_006.png> from <data/images/button_white_animated.zip>
    [WARNING] [Image       ] Unable to load image<buttonoverlay_dn_006.png> in zip <data/images/button_white_animated.zip> trying to continue...
    [DEBUG  ] [ImageImageIO] Load <buttonoverlay_dn_007.png> from <data/images/button_white_animated.zip>
    [WARNING] [Image       ] Unable to load image<buttonoverlay_dn_007.png> in zip <data/images/button_white_animated.zip> trying to continue...
    [DEBUG  ] [ImageImageIO] Load <buttonoverlay_dn_008.png> from <data/images/button_white_animated.zip>
    [WARNING] [Image       ] Unable to load image<buttonoverlay_dn_008.png> in zip <data/images/button_white_animated.zip> trying to continue...
    [DEBUG  ] [ImageImageIO] Load <buttonoverlay_dn_009.png> from <data/images/button_white_animated.zip>
    [WARNING] [Image       ] Unable to load image<buttonoverlay_dn_009.png> in zip <data/images/button_white_animated.zip> trying to continue...
    [DEBUG  ] [ImageImageIO] Load <buttonoverlay_dn_010.png> from <data/images/button_white_animated.zip>
    [WARNING] [Image       ] Unable to load image<buttonoverlay_dn_010.png> in zip <data/images/button_white_animated.zip> trying to continue...
    [DEBUG  ] [ImageImageIO] Load <buttonoverlay_dn_011.png> from <data/images/button_white_animated.zip>
    [WARNING] [Image       ] Unable to load image<buttonoverlay_dn_011.png> in zip <data/images/button_white_animated.zip> trying to continue...
    [DEBUG  ] [ImageImageIO] Load <buttonoverlay_dn_012.png> from <data/images/button_white_animated.zip>
    [WARNING] [Image       ] Unable to load image<buttonoverlay_dn_012.png> in zip <data/images/button_white_animated.zip> trying to continue...
    [DEBUG  ] [ImageImageIO] Load <buttonoverlay_dn_013.png> from <data/images/button_white_animated.zip>
    [WARNING] [Image       ] Unable to load image<buttonoverlay_dn_013.png> in zip <data/images/button_white_animated.zip> trying to continue...
    [DEBUG  ] [ImageImageIO] Load <buttonoverlay_dn_014.png> from <data/images/button_white_animated.zip>
    [WARNING] [Image       ] Unable to load image<buttonoverlay_dn_014.png> in zip <data/images/button_white_animated.zip> trying to continue...
    [DEBUG  ] [ImageImageIO] Load <buttonoverlay_dn_015.png> from <data/images/button_white_animated.zip>
    [WARNING] [Image       ] Unable to load image<buttonoverlay_dn_015.png> in zip <data/images/button_white_animated.zip> trying to continue...
     Traceback (most recent call last):
       File "main.py", line 147, in <module>
         mainApp().run()
       File "/Library/Python/2.7/site-packages/kivy/app.py", line 527, in run
         root = self.build()
       File "main.py", line 140, in build
         upl = mainclass()
       File "main.py", line 76, in __init__
         background_normal = 'data/images/button_white_animated.zip')
       File "/Users/Eric/Projects/kivy/examples/widgets/sequenced_images/uix/custom_button.py", line 42, in __init__
         keep_ratio = self.keep_ratio, mipmap = True)
       File "/Library/Python/2.7/site-packages/kivy/uix/image.py", line 219, in __init__
         self.texture_update()
       File "/Library/Python/2.7/site-packages/kivy/uix/image.py", line 234, in texture_update
         nocache=self.nocache)
       File "/Library/Python/2.7/site-packages/kivy/core/image/__init__.py", line 418, in __init__
         self.filename = arg
       File "/Library/Python/2.7/site-packages/kivy/core/image/__init__.py", line 606, in _set_filename
         mipmap=self._mipmap, nocache=self._nocache)
       File "/Library/Python/2.7/site-packages/kivy/core/image/__init__.py", line 339, in load
         return ImageLoader.zip_loader(filename)
       File "/Library/Python/2.7/site-packages/kivy/core/image/__init__.py", line 276, in zip_loader
         raise Exception('no images in zip <%s>' % filename)
     Exception: no images in zip <data/images/button_white_animated.zip>

However, wrapping the: 

    im = loader(tmpfile, **kwargs)

in `try...except` was not enough.

Sometimes there was an image not recognized by any loader. (e.g. an artefact from the zip program while performing the zipping), and if that artefact turns out to be the last file attempted to be loaded, `im` in:

    im._data = image_data

would be `None`, causing an AttributeError.

So I've had to change it to use another variable `image` to keep track of the last image successfully loaded. It is only assigned when `im` returned from the loader is not `None`.