swc-installation-test-2.py: Raise an error on empty version stream

This makes the upcoming mult-path checking easier to implement.
swc-installation-test-2.py: Use _ROOT_PATH for MS Windows compatability

I don't think starting a path with os.sep gives you a root path on
Windows.  We need the backslash in _ROOT_PATH, because:

  os.path.join('c:', 'foo')

gives 'c:foo', a path relative to the current directory on drive C: [1].

[1]: http://docs.python.org/3/library/os.path.html#os.path.join
swc-installation-test-2.py: Add CommandDependency.paths fallbacks

The old CommandDependency implementation assumed that commands would
exist in one of the directories listed in PATH.  This assumption is,
unfortunately, often not satisfied on MS Windows or OS X.  In order to
work around this problem, you can now store a list of fallback paths
that will be tried if there is an error extracting the version stream
from the bare CommandDependency.command.

I've added fallback paths for Notepad++, Firefox, and Google Chrome,
as reported by Ethan White for Windows 7.