Raise exception when sniffer did not reply on serial commands

[totyz]

fix: Raise exception instead of error log when sniffer did not reply properly to setup commands

When sniffer did not reply properly to setup commands, it's useless. Thus, is better to inform finish work instead of keeping it running. The exception is in serial_reader thread so, additional method is_running has been implemented to give user ability to confirm that sniffer is up and running after startup.

Signed-off-by: Tomasz Tyzenhauz tomasz.tyzenhauz@nordicsemi.no

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