Add error checks for all I2C reads and writes and return error values

[waynepiekarski]

Currently if the device is not connected or if any of the I2C operations fail, the library will receive all zeros, continue with the calculations, and then return incorrect values. Previously it was not possible to return an error code from read() either, so this has been added as well.

[waynepiekarski]

@patrickelectric I was hoping you could review this code which fixes some bugs in the sensor error handling