swc-installation-test-2.py: Print full DependencyError chain

swc-installation-test-2.py: Print full DependencyError chain

If a requirement is not satisfied due to an and_dependency or
or_dependency failure, raise a new DependencyError and attach the
original error as its cause (this would be easier if we didn't have to
preserve Python 2.x support).  This way we can print the whole chain
(e.g. virtual-shell failed because sh is not installed), instead of
just the lowest level error.

Also indent the messages, because:

  check for virtual-editor failed:
    or-dependency not satisfied for virtual-editor
    For instructions on installing an up-to-date version, see
    http://software-carpentry.org/setup/
    cause:
    check for Notepad++ (notepad++) failed:
      could not find 'notepad++' executable
      For instructions on installing an up-to-date version, see
      http://software-carpentry.org/setup/

is more readable than:

  check for virtual-editor failed:
  or-dependency not satisfied for virtual-editor
  For instructions on installing an up-to-date version, see
  http://software-carpentry.org/setup/
  cause:
  check for Notepad++ (notepad++) failed:
  could not find 'notepad++' executable
  For instructions on installing an up-to-date version, see
  http://software-carpentry.org/setup/