The existing implementation does not provide a useful error message
if a negative offset is passed in File.ReadAt or File.WriteAt. This
change is to return descriptive errors. An error of type *PathError
is returned to keep it consistent with rest of the code.

There is no need to add an exported error variable since it's used only
in one file.

Fixes #19031

Change-Id: Ib94cab0afae8c5fe4dd97ed2887018a09b9f4538
Reviewed-on: https://go-review.googlesource.com/39136Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>