Correct GlanceStoreException to provide valid message - Glance

This is the relevant change on glance side. The main change on
glance_store side is Ic4423405f831088fb51de7cfdf1243e9a0ff32ea ,
which be used to allow glance_store exception provide a proper error
message instead of None message all the time.

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

Change-Id: I0d6aa29bca3809da4c71d278ef17897eae15ee6c
Signed-off-by: Zhi Yan Liu <zhiyanl@cn.ibm.com>
Merge "Correct GlanceStoreException to provide valid message - Glance"