Potting the 9 ball in a foul shot - what happens?

TL;DR: The ball is returned to the table and you do not automatically loose

The National Billiard Association has these rules:

2.6 Spotting Balls

If the nine ball is pocketed on a foul or push out, or driven off the table, it is spotted. (See 1.4 Spotting Balls.) No other object ball is ever spotted.

A spotting ball is defined as:

1.4 Spotting Balls

Balls are spotted (returned to play on the table) by placing them on the long string (long axis of the table) as close as possible to the foot spot and between the foot spot and the foot rail, without moving any interfering ball. If the spotted ball cannot be placed on the foot spot, it should be placed in contact (if possible) with the corresponding interfering ball. However, when the cue ball is next to the spotted ball, the spotted ball should not be placed in contact with the cue ball; a small separation must be maintained. If all of the long string below the foot spot is blocked by other balls, the ball is spotted above the foot spot, and as close as possible to the foot spot.