Deleting files made QA tool error out with odd exception message

[davelab6]

google/fonts#2674 failed @ https://travis-ci.org/github/google/fonts/builds/725453523 with this:

Running gftools qa
Traceback (most recent call last):
  File "/home/travis/virtualenv/python3.7.1/bin/gftools-qa.py", line 549, in <module>
    main()
  File "/home/travis/virtualenv/python3.7.1/bin/gftools-qa.py", line 472, in main
    ignore_static_dir=False,
  File "/home/travis/virtualenv/python3.7.1/lib/python3.7/site-packages/gftools/utils.py", line 138, in download_files_in_github_pr
    results += download_files_in_github_dir(url, dst, overwrite=False)
  File "/home/travis/virtualenv/python3.7.1/lib/python3.7/site-packages/gftools/utils.py", line 181, in download_files_in_github_dir
    files = [f for f in repo.get_contents(url.dir, ref=url.branch)
  File "/home/travis/virtualenv/python3.7.1/lib/python3.7/site-packages/github/Repository.py", line 1755, in get_contents
    parameters=url_parameters,
  File "/home/travis/virtualenv/python3.7.1/lib/python3.7/site-packages/github/Requester.py", line 319, in requestJsonAndCheck
    verb, url, parameters, headers, input, self.__customConnection(url)
  File "/home/travis/virtualenv/python3.7.1/lib/python3.7/site-packages/github/Requester.py", line 342, in __check
    raise self.__createException(status, responseHeaders, output)
github.GithubException.UnknownObjectException: 404 {"message": "Not Found", "documentation_url": "https://docs.github.com/rest/reference/repos#get-repository-content"}
The command "bash .ci/run.sh" exited with 1.

It was an unusual PR, so I merged it anyway, but this exception should be handled more informatively :)