Code:
/**
 * This function gets called automatically if there was an I2C Error like
 * the slave sent a "not acknowledge" (NACK, error codes e.g. 0x20 or 0x30).
 * The most common mistakes are: 
 *   - using the wrong address for the slave
 *   - slave not active or not connected to the I2C-Bus
 *   - too fast requests for a slower slave
 * Be sure to check this if you get I2C errors!
 */