Reject invalid epoch requests

[rillian]

#15 allows the client to request a specific epoch, which I think is appropriate, since it must coordinate that with its submission preparation. It also adds error propagation if the randomness evalutation fails, e.g. if the epoch has already been punctured.

The server should also reject requests for epochs that are part of the ppoprf object config, but not yet "in service" to limit the time window available for sybil attacks. It might as well also reject known-expired epochs in the request validator instead of relying on the slower randomness_server_eval() call.

[rillian]

This was resolved by #50.