Bad Temperature readings do not update correctly

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A bad temperature reading updates to 0C instead of showing as an invalid number (XX or the like).

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To clarify, this means that the temperature sensor is missing from the controller and reads as 0C on the website.

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It is noted that commit MAGLaboratory/haldor@cd1a0e7 changed the handling of a missing temperature sensor from "--" to "XX" which may or may not have changed the website behavior.

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it should be further noted that development efforts made did not correct this problem completely.

The JS updater now turns XX into NaN when updating the synoptic view under temperature sensor failure conditions.

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JS updater still turns XX into NaN.