Fix panic if qemu-nbd is missing

Running 'capstan run' on Darwin that does not have qemu-ndb installed
panics:

  mavericks:capstan-example penberg$ capstan run
  Building capstan-example...
  exec: "qemu-nbd": executable file not found in $PATH
  exec: "qemu-system-x86_64": executable file not found in $PATH
  dial tcp 127.0.0.1:10000: connection refused
  panic: runtime error: invalid memory address or nil pointer dereference
  [signal 0xb code=0x1 addr=0x0 pc=0x4751d]

Fix that by making SetArgs propagate the error to the callers.

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>