Make raise/re-raise of exceptions in the code consistent

Re-raising exceptions across the code is somewhat skewed. At some places an
exception is handled and re-raised, at other places a plain raise statement is used and the
correct exception context may not be clear from the code.

Such statements should be identified and refactored to use the right contextual exception
object during a re-raise and supplement with a LOG message if needed.

Example from current trunk:
1.
        except Exception:
            try:
                with excutils.save_and_reraise_exception():
                    LOG.exception(_('Failed to get nw_info'),
                                  instance=instance_ref)
            except Exception:
                if ignore_missing_network_data:
                    return
                raise # Specify the exception object that should be re-raised in this context

https://github.com/openstack/nova/blob/master/nova/notifications.py#L237

2. except OSError as e:
        if e.errno == errno.ENOENT:
            return
        else:
            raise # will be clearer if the caught object 'e' is passed as an expression here

https://github.com/openstack/nova/blob/master/nova/utils.py#L453