Convert non-English exception message when a store loading error

In non-English environment when a store driver loading failed then a
BackendException will be raised with a non-ascii message. So we need
convert
any coded exception message to unicode string for logging.

This patch fixed a minor defect in
https://review.openstack.org/#/c/37421/
And the test case has be enhanced also.

Fixes bug 1202098

Related-Id: I9a58393b599aacc427cb4135988ecf5c3e90e455
Change-Id: I1cdc71537fc1205e5fa4ff9744b441e5d88c1d04
Signed-off-by: Zhi Yan Liu <zhiyanl@cn.ibm.com>
Merge "Convert non-English exception message when a store loading error"