pmbootstrap.py: Don't print stacktrace when KeyboardInterrupt is caught

This looks obnoxious and I cannot think of a single time I found it useful, so let's make it prettier.

One thing I'm unsure about is what exit code to set. Currently pmbootstrap appears to exit "normally" (0) with this, which may not be ideal if you're using pmbootstrap in a script. However, I'm not sure what the exit code convention for pmbootstrap is, or which ones are available, so suggestions are more than welcome.