It's gonna crash in `virgo`. Shall we explicitly throw an error explaining it shouldn't be `None`?

```
Updating git submodules....
Creating GYP include files (.gypi)
Bundling from /private/tmp/virgo
Traceback (most recent call last):
  File "./configure", line 7, in <module>
    base.configure.main()# bundle_list_file="bundle.list")
  File "/private/tmp/virgo/base/configure.py", line 349, in main
    platform['variables']['BUNDLE_LIST_FILE'] = os.path.abspath(bundle_list_file)
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/posixpath.py", line 343, in abspath
    if not isabs(path):
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/posixpath.py", line 53, in isabs
    return s.startswith('/')
AttributeError: 'NoneType' object has no attribute 'startswith'
```