Try and detect a serialize function inside arbitrary preprocessor blocks and throw an error
Make a couple of other "can't have things inside preprocessor blocks" messages consistent with this one
#rb steve.robb
#lockdown ben.marsh
#ROBOMERGE-SOURCE: CL 4408769 in //UE4/Main/...
#ROBOMERGE-BOT: RELEASE (Main -> Release-Staging-4.21)
[CL 4408774 by graeme thornton in Staging-4.21 branch]