Include original exception in ClassNotFound exception
Include original exception in ClassNotFound exception

By doing this, we allow the error messages to be more useful. When an import
of a class fails because of a missing module dependency, it would have fail
that way for example:

$ nova-manage
Traceback (most recent call last):
  File "./bin/nova-manage", line 84, in <module>
    from nova import image
  File "/home/jd/Work/src/nova/nova/image/__init__.py", line 22, in <module>
    from nova.image import glance
  File "/home/jd/Work/src/nova/nova/image/glance.py", line 42, in <module>
    GlanceClient = utils.import_class('glance.client.Client')
  File "/home/jd/Work/src/nova/nova/utils.py", line 66, in import_class
    raise exception.ClassNotFound(class_name=class_str)
nova.exception.ClassNotFound: Class Client could not be found

This does not help the user, since it indicates the class Client cannot be
found, even if it is actually found but fail to import.

With this commit, the error message is better:
nova-manage
Traceback (most recent call last):
  File "./bin/nova-manage", line 84, in <module>
    from nova import image
  File "/home/jd/Work/src/nova/nova/image/__init__.py", line 22, in <module>
    from nova.image import glance
  File "/home/jd/Work/src/nova/nova/image/glance.py", line 42, in <module>
    GlanceClient = utils.import_class('glance.client.Client')
  File "/home/jd/Work/src/nova/nova/utils.py", line 66, in import_class
    raise exception.ClassNotFound(class_name=class_str, exception=exc)
nova.exception.ClassNotFound: Class Client could not be found: No module named kombu.connection

This helps to know that in this kombu is missing.

It would probably even better to rename ClassNotFound to
ClassCannotBeImported or something like that too.

Change-Id: I4100d931a0a825fa0729d5467d2f9268fdd2a261
Signed-off-by: Julien Danjou <02b9c1d548d177421b793c726ed68b7aac9b5a66@enovance.com>

n0ano/ganttclient