New account key is not preserved on failure

[Lekensteyn]

When an account key is created, but the authorization failed (because the HTTP server is not reachable for example), the account key key is not stored. It should probably persist this key even if it failed to generate a certificate.

Edit: what about allowing simp_le to generate just the account key and nothing else? I.e. allow simp_le.py -f account_key.json to generate the account key. Then a wrapper script can ensure that this file is available and store it appropriately.