If a multibyte string without unicode prefix 'u' is passed to
NeutronException in python 2, string subsitution will fail.
In python 2, multibyte string without 'u' prefix is encoded into bytes.
As a result a string substitution in NeutronException.__init__ will fail.
Change-Id: I1aa08b69fe119087a7a49fda768a24f85f0e7c76
Closes-Bug: #1235228