aspen wrongfully catches IOErrors that occur within configuration scripts

This is to correct issue #212 that I just created. I also added a test for this in `tests/test_configuration.py`. The test uses an additional config file at `tests/_config_test.py` to trigger the error (the underscore prevents it from being run as a normal test file). This seems to me as a rather rough way of doing it, so just let me know if you'd rather me do it in another way.