Missing handler for socket error/dead caused by weak signal strength

[CptHolzschnauz]

What:
A created socket can 'die' out of a sudden when the server does simply not answer anymore without closing the socket.
Then a +USOWR command can stop in the middle of the write process and the modem answer with +CME Error 3 (Operation not allowed). A error readout with +USOER says "65 EEOF - End of file."

The client wants to write data into a dead socket, the bytestream is blocked because the server does simple not read anymore.

Result: The board freezes because the NbClient can't handle that error.

Remedy: these processes take place on the baseband and cannot be influenced or predicted. however, the nbclient must be able to handle the error message of the modem cleanly.

[CptHolzschnauz]

closed with #84