Print error and abort when reading/writing fails
Print error and abort when reading/writing fails

Not really necessary but considering that the project is young,
additional messages could help debugging issues.

uraimo/SwiftyGPIO,colemancda/SwiftyGPIO,stephanmantler/SwiftyGPIO,uraimo/SwiftyGPIO,stephanmantler/SwiftyGPIO