Problem: cmake won't error if Find<LIB> fails
Problem: cmake won't error if Find<LIB> fails

Solution: error out manually if <LIB>_FOUND is false and the
dependency is not optional

taotetek/zproject,keent/zproject,paddor/zproject,zeromq/zproject,oikosdev/zproject,miska/zproject,eburkitt/zproject,taotetek/zproject,eburkitt/zproject,hintjens/zproject,paddor/zproject,trevorbernard/zproject,twhittock/zproject,hintjens/zproject,taotetek/zproject,twhittock/zproject,asokoloski/zproject,paddor/zproject,keent/zproject,zeromq/zproject,asokoloski/zproject,zeromq/zproject