Correct GlanceStoreException to provide valid message - glance_store

This fix allows glance_store exception provide a proper error message
instead of None message all the time.

This is the main change on glance_store side. The relevant change on
glance side is I0d6aa29bca3809da4c71d278ef17897eae15ee6c .

Note, once this change on glance_store side get merged, we need to land
the change for glance before release new glance_store.

Change-Id: Ic4423405f831088fb51de7cfdf1243e9a0ff32ea
Signed-off-by: Zhi Yan Liu <zhiyanl@cn.ibm.com>
Merge "Correct GlanceStoreException to provide valid message - glance_store"