v0.50.0

- Treat HTTP 4xx errors as fatal for SVR connections. In Java, these
  result in a new NetworkProtocolException, distinct from the
  NetworkException that represents a potentially-transient error.

- Java: Fix missing AttestationFailedException in throws clauses

- Update backup validation

- net: Back off DNS requests like we do for the main connection

- net: Chat responses that come in just before the connection is
  closed won't be dropped as aggressively.