While I have heard of a few D5000 users encountering
Error. Press shutter release button again., indicating a shutter failure, this is rare. Considering your usage, the shutter should last at least two to three years. If and when the shutter fails, it would probably cost about $200 to $300 to replace, but that's a large part of the value of the camera body and you may want to get a new camera at that point.
Note that Nikon says "tested to over 100,000 cycles" (emphasis added), so the shutter should be expected to last some time past the 100K mark and probably should not be expected to fail at just over 100K, but likely somewhere between 120K and 150K. Furthermore, this is a mean time between failures; your shutter may last longer or shorter than this number of cycles. In fact, there have been brand new D5100s with this problem.