Re-raise exception of unloadable library

The patch re-raises the exception when a stevedore extention fails to
load.

Before this patch, the config generator silently ignore options
from a library (-l) when the oslo.config.opts entry point of the
library fail to load, and result to an incomplete configuration file.

Closes bug #1287789

Change-Id: I58ce4f0b0c422c558f2465fb008ce72b7dffc50f