Middleware: Key not found returns Internal Server Error

[muhlemmer]

At the moment, when a public key is not found an (incorrect) internal server error is returned to the client. gRPC already returns the correct "not found" class code, but it is unhandled by middleware.

Instead of internal server error, the client should be redirected for login.

Normally, public keys are never deleted from the database. New ones are just added at server start and older ones can always be obtained. Therefore, this bug is not very critical.