Report offending file for invalid require

It's always been a surprise to me that Racket only says that it couldn't require X, but doesn't say where X is required, making it difficult to find the problem and fix it.

E.g.,

;; a.rkt

#lang racket

(require "b.rkt")

;; b.rkt

#lang racket

(require "c.rkt")

(c.rkt doesn't exist)

Running a.rkt, I get:

../../Applications/Racket v7.3/share/pkgs/gui-lib/mred/private/snipfile.rkt:324:2: open-input-file: cannot open module file
  module path: c.rkt
  path: c.rkt
  system error: no such file or directory; rktio_err=3
  no package suggestions are available .

I expect that b.rkt shows up in the error message so that users know that it should be fixed there.

Reproduce the failure using the a.rkt, b.rkt, and missing c.rkt example, then inspect the reported error path at snipfile.rkt:324. Trace how the require of c.rkt is reported when it originates in b.rkt; done means the error identifies b.rkt as the offending requiring file.