Handling of spanloss > 27 dB

When a spanloss of 28 dB or higher is measured, the spanloss is not set. This causes a later service-power-setup to fail (when the setPower function in PowerMgmt.java cannot get the spanloss to calculate target power value) with the controller returning a 500 Internal Server Error. Also, when this happens, there are no helpful log messages for understanding what went wrong. I know that a spanloss > 27 db is outside the OpenROADM spec but maybe it would be better to anyway set the spanloss and then let the power setup fail with some helpful log message.