App should return P4RT error to PFCP agent when switch tables are full

[ccascone]

We run into issues were the PDR and FAR tables on Tofino were full, but the up4 app was accepting new entries from the PFCP agent.

Instead, the app should be able to check whether tables are full and return errors to the PFCP agent when new entries cannot be accommodated. The PFCP agent should be responsible for relaying errors back to the SPGW-C.

Errors can be reported via gRPC status as described in the P4RT spec.