Include request_id in handshake error responses

[almstrand]

Fixes #838

When an exception is thrown inside handshake (server/src/auth.js) it is currently caught in error_wrap_socket (server/src/client.js) where the originating request's request_id is unknown. Failed handshake requests will therefore respond with request_id: null

This change is to instead return a rejected promise, which will be caught in the catch clause inside handle_handshake (server/src/client.js), from where a JSON response is sent including the request_id.

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This change is