Pageloader: Differentiate internal errors and page not found errors

So that we can show proper error message at client side.
Errors from parsoid page load is considered as 404s for CX
page load API.

Bug: 67468
Change-Id: Iefd78259a172e930109d24ec67192f1f87b21002
Merge "Pageloader: Differentiate internal errors and page not found errors"