Better error checking on private key and csr.  Also, pull in file names ...

...instead of pointers.
`:raises Exception:` to be consistent with the rest of the code. But... please use `letsencrypt.errors.client.LetsEncryptClientError` instead.

To be honest, I've been using wrong syntax myself: `:raises: FooException ...`. Thanks for complying with the Sphinx doc, I'll fix my mess in follow-up PR!