The controller's attention might be elsewhere (have you never been sent
right through a localizer?). That said, my Garmin 196 does warn me
when it loses reliable reception, though it's not proper RAIM.

Reliable reception is not equivalent to integrity. You can have great
reception and one bad signal that drives your position off hundreds of
miles. And yes it is rare (10E-5/hour). BUt aviation integrity is at
the 10E-7 rate.

Ron Lee