Enable unicode error message

Keystone exceptions could only take byte string message as the
message arguments to construct exception instances because of
the way its super class StandardError implements __unicode__.
This patch can also make sure it would not unintentionally remove
line breaks and indentation in a explicitly given message argument.

Fixs bug #1168879

Change-Id: I7916efc87845cfc4dba705e9474125b275affc13
Merge "Enable unicode error message"