Improve pydeps related errors

This adds a crbug comment next to the gn error that arises when a pydeps
file is missing. This also improves UX when PRESUBMIT.py encounters an
empty pydeps file. Instead of using the command in the pydeps file
itself (normal case), if the file is empty, it generates an educated
guess of a command that should work in most cases.

Bug: 1102058
Change-Id: Ida016c1efe5a4d3318f196e5d95ab31fd8e5d955
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2281168
Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Commit-Queue: Mohamed Heikal <mheikal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#785496}