Adopt error API to convey server errors to the client

https://github.com/apple/swift-homomorphic-encryption-protobuf/pull/21 added a new error message to protobufs.

pir-service-example should adopt that.

For example, it can return the new configuration to the devices when device makes a query with invalid configuration. (https://github.com/apple/pir-service-example/issues/94#issuecomment-3239914507)

Start with the protobuf error-message change in PR #21 and the invalid-configuration flow referenced in issue #94's comment. Trace how pir-service-example sends server errors to devices; done means the new error API can convey the configuration response for an invalid device query.

Written by the indexing model from the issue text.