pk: Don't print backtrace on mount error
pk: Don't print backtrace on mount error

- fuse_mount() may fail for various reasons: kernel module not loaded,
bad permissions on /dev/fuse, etc.  Don't print a backtrace in this case.

cmusatyalab/openisr,cmusatyalab/openisr,cmusatyalab/openisr,cmusatyalab/openisr,cmusatyalab/openisr