Let AssertionErrors bubble up

Sometimes we want to use AssertionError to cause test
failures, and it is unlikely to be used in real code,
so just let the error bubble up if it is encountered
while loading a plugin.

Change-Id: I7ad20b0796c1a02427d324aa7f35bb238013c425
Signed-off-by: Doug Hellmann <doug.hellmann@dreamhost.com>

docs/source/history.rst

@@ -2,6 +2,11 @@
  History
 =========
 
+dev
+
+  - Ignore AssertionError exceptions generated when plugins are
+    loaded.
+
 0.7.2
 
   - Fix logging support for Python 2.6.

stevedore/extension.py

@@ -76,7 +76,7 @@ class ExtensionManager(object):
                                             )
                 if ext:
                     extensions.append(ext)
-            except KeyboardInterrupt:
+            except (KeyboardInterrupt, AssertionError):
                 raise
             except Exception as err:
                 LOG.error('Could not load %r: %s', ep.name, err)