ReactMount should not optimistically traverse unknown nodes

From #2005, superseded by #2262

I think it's a mistake to optimistically traverse unknown nodes. The only really valid/common use is implicit `tbody`, but even that is a rather bad idea (#2005). It can also hide important errors (don't mess with siblings of React nodes). We're better off telling the user something is wrong and improving the error message the user receives (#1987).

Test plan: `grunt test`