Allow creation of an empty OutputCollection so that the object can be handed
around in driver.py for higher-level document wrangling.
Also, repair one inobvious statement, thank you pep8/pyflakes
should standardize on IOError for errno.ENOENT across the package
adhere to standard choice of logging message
make sure to skip a non-directory in OutputCollection ("continue")
an OutputTree must exist already, contains all of the OutputDocuments
an OutputDocument determines the stem from the dirname and depends on
the OutputNamingConvention for choosing names for the primary files of each
generated output type