ensure unique exit codes for atclient_get_shared_encryption_key_shared_by_me

[XavierChanth]

A bunch of return codes are propagated from other functions and in some cases those functions are propagating error codes from other libraries.

This particular function has a special case error code which represents:

error:AT0015-key not found

This is the only error code in which it is acceptable to create a new shared encryption key, so we need to guarantee that the return code for error:AT0015-key not found can only occur when we receive error:AT0015-key not found from the server.