module recorded as "unreadable" when one of its direct dependencies is unloadable

[rchen152]

Reproduce with:

git clone git@github.com:GoogleCloudPlatform/django-cloud-deploy.git
cd django-cloud-deploy
pip install .
importlab --tree django-cloud-deploy/cloudlib/database.py

importlab reports:

Reading 1 files
Source tree:

Unreadable files:
  /usr/local/google/home/rechen/django-cloud-deploy/django_cloud_deploy/cloudlib/database.py

In fact, database.py is readable, but a file in one of its pip-installed dependencies (django/contrib/auth/models.py) throws an error at import time.

• importlab should mark models.py instead of database.py as unreadable.
• It would be useful to surface the actual error that occurred, to give the user a better idea of why the file is unreadable.