This drops testfiles.ReadOrDie and updated testfiles.Exists to return an
error, forcing the caller to decide whether to call framework.Fail or do
something else.
It makes for a slightly less friendly API, but also means the package is
decoupled from framework again, as per the comments at the top of the
file
Every caller of ReadOrDie() specified ginkgo.Fail as fail argument,
and that was intentional to avoid depending on Ginkgo.
However that just spreaded the dependency on Ginkgo to caller sides.
Especially that was unnecessary e2e test framework "ingress" depended
on Ginkgo only for the above reason.
Now we are cleaning up the dependencies on e2e tests, so let's just
remove such dependencies.
