The OSError thrown by I2C doesn't have errno in args[0]

[deshipu]

In particular, when there is no ACK response to a start condition and address, it should raise OSError with errno=19 (ENODEV), otherwise it's impossible to distinguish the different kinds of errors.

I need this for the SI7021 sensor driver: that sensor signals its busyness by sending NACK to I2C requests until it finished the measurements, so I need to be able to tell this case from other I2C errors.

This works correctly in the latest version of MicroPython.

[deshipu]

In addition, i2c.readfrom_into() doesn't even raise an exception when it receives a NACK (i2c.writeto() does).