fix #23: loadImages throws error instead of attempting to call exit()
fix #23: loadImages throws error instead of attempting to call exit()

- there is no exit() function in node or browser, it's inappropriate to
  call process.exit
- future update might be necessary to do some cleanup after error is
  thrown?

ericblade/quaggaJS,ericblade/quaggaJS,ericblade/quaggaJS,ericblade/quaggaJS