[keyserver] improve error message on identity connection failure

Summary: the existing error message, "Unable to connect to identity service," is not very helpful

Test Plan:
tried to connect to my local identity service which was not running. Saw this error in the keyserver logs:

[NODEM] Failed to register user: Transport Error: tonic::transport::Error(Transport, hyper::Error(Connect, ConnectError("tcp connect error", Os { code: 61, kind: ConnectionRefused, message: "Connection refused" })))

much better than just "Unable to connect to identity service"

Reviewers: will

Reviewed By: will

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D11145