Use "security sensitive" buzzword for errorcode fields

GSSNegotiate and CombineTokens have errorcode fields in their return
structures, in order to hold security sensitive error codes.
That is, codes which will change the security behavior of the client
that receives them.  Mention this justification where these fields are used.

Change-Id: I9f02391d2fca8ce5f0159d95688968c963bf2ac0