add generic_nack

[komuw]

from smpp spec section 4.3

A generic_nack response is returned in the following cases:
• Invalid command_length
If the receiving SMPP entity, on decoding an SMPP PDU, detects an invalid
command_length (either too short or too long), it should assume that the data is corrupt. In
such cases a generic_nack PDU must be returned to the message originator.
• Unknown command_id
If an unknown or invalid command_id is received, a generic_nack PDU must also be
returned to the originator.

[komuw]

maybe we should add it here;

naz/naz/client.py

Line 985 in 7abf591

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