Fix error: module 'os' has no attribute 'mkdirs'

[c-w]

Without this fix, the initialize_backend method fails with the error being silently swallowed by the broad except block.

In order to avoid these sorts of exception masking issues going forward, commit ff68452 also removes the except blocks that wrap the os.makedirs calls in favor of the exist_ok flag so that the only exceptions that get ignored are ones related to the directory already existing. The exist_ok argument was added in Python 3.2 (source). Given that this project supports Python 3.5+, it should be fine to use this flag.