Unhandled error when computername not set on Mac OS

Hi,

It seems that when the ComputerName is not set in Mac OS, the library fails in an unexpected way because scutil does not return a result. Please see this stackoverflow question:

http://stackoverflow.com/questions/36155236/nodejs-error-typeerror-cannot-read-property-stdout-of-undefined

Perhaps the code should throw if a `err` has a value?
