Throw MalformedBundleException on broken zip files

sugar.bundle.bundle.Bundle users (e.g.
jarabe.model.bundleregistry.BundleRegistry._add_bundle) only expect
MalformedBundleException to be thrown, not exceptions of zipfile.

This patch solves the most severe issue in #1876: filling up the
filesystem with temporary files that won't be deleted afterwards.

Before we can consider this bug completely fixed, we still need
to do something for the remaining issues:

 1) Unpacking shouldn't be attempted if there isn't a safety margin

 2) System becomes unresponsive during unpacking

 3) No progress indication for the operation, so users are tempted
    to click multiple times

 4) No error messages displayed for unpacking errors, which is a
    common Sugar nuisance.

http://bugs.sugarlabs.org/ticket/1876#comment:5 offers possible
strategies for (1) and (2).

Signed-off-by: Martin Dengler <martin@martindengler.com>
Signed-off-by: Bernie Innocenti <bernie@codewiz.org>
[style fixes, adjusted description]
Signed-off-by: Sascha Silbe <silbe@activitycentral.com>
Reviewed-by: Aleksey Lim <alsroot@activitycentral.org>

src/sugar/bundle/bundle.py

@@ -71,7 +71,11 @@ class Bundle(object):
         self._zip_file = None
 
         if not os.path.isdir(self._path):
+            try:
                 self._zip_file = zipfile.ZipFile(self._path)
+            except zipfile.error, exception:
+                raise MalformedBundleException('Error accessing zip file %r: '
+                                               '%s' % (self._path, exception))
             self._check_zip_bundle()
 
         # manifest = self._get_file(self._infodir + '/contents')