Using unicode() convert non-English exception message

In non-English environment when the store driver loading failed, the
BackendException will be raised with a non-ascii message. We need call
unicode() to convert the message before using it.

Fixes bug 1202098

Change-Id: I9a58393b599aacc427cb4135988ecf5c3e90e455
Signed-off-by: Zhi Yan Liu <zhiyanl@cn.ibm.com>
Merge "Using unicode() convert non-English exception message"