swallow error when copying sys.config and vm.args, which are not used in...

I'm trying rebar release and upgrade in ejabberd, which will always exit due to the anonying badmatch error. The reason is sys.config and vm.args are not used in ejabberd but rebar still copy them when generating package. 

Related command is:
```
./rebar generate-upgrade previous_release=ejabberd_1 -vvv
```

I'm now using ejabberd_update to do the upgrade but don't like to miss the rebar function.