Wire.write ignores twi_transmit return values

Failures from twi_transmit are ignored in Wire.write when sending in slave mode. This coupled with the fact that the function always returns the quantity given, prohibits check for failure to send data.

Start by locating Wire.write and twi_transmit in the Arduino AVR core and read the slave-mode send path. Determine how twi_transmit failures should affect the reported quantity, then verify that Wire.write no longer claims all data was sent when transmission fails.

Written by the indexing model from the issue text.