After replacing the left front wheel bearing on a 2003 ford focus, the abs and traction warning light stay on

The bearings on the wheels has to be installed one way only, the rotor ring is built into the bearing and if installed the wrong way round the sensor cant pick up the wheel speed causing the ligt to be switched on in the cluster. The diagnostic equipment should pick this up as a fault once the car has been driven, it will see the sensor when you do the diagnostic if the car is standing still but once the car is moving and no signal is send from that wheel it would register a fault into the memory of the PCM that you would be able to retrieve later.
There is a way for you to test the sensor yourself by using a multimeter set on volts, you jack the vehicle up at that wheel, install a vehicle stand to support the vehicle to be safe, disconect the wheel speed sensor and probe the two wires into the sensor, spin the wheel by hand and a small amount of current will be produced meaning the the sensor is working and the wheel bearing is installed the correct way, you can also do a continuaty test on the sensor by switching the multimeter onto ohms, but be very carefull, if this sensor has three wires you cant do the continuaty test because of the integrated circuit in the sensor, the three wire sensors opperate on a voltage from 5Volt and it would be destroyed by the multimeter opperating on 9Volt.
If the continuaty test passes and the bearing is installed the correct way, the most comon fault would be a break in the wiring leading to the PCM
Hope this will be helpfull.