Need to differentiate between 'no target found' and 'lost stream'

[ajtgarber]

Currently if the camera finds no targets, time_since_valid is set to -1 and for each frame time_since_valid is -1 the code will continue to reacquire the image stream.

Suggested fix: set time_since_valid = -1 when the stream is lost and set it to -2 when there's no target found, on the robot side of things the coordinates can be ignored if the value is negative (doesn't matter why). The handle_times function determines when the stream is lost.