Gracefully error out when VirtualBox doesn't bring up the vboxnet0 interface

[AlBaker]

In some cases, like an OSX update, the vboxnet0 interface can disappear. A re-install of VirtualBox appears to be the only way to get it back (unless someone knows otherwise). Perhaps this is solved in 4.3, but either way - the vmfest host function should error out more gracefully as unsuccessful node creation and perhaps give a reason. Right now it's a null pointer exception.

Error is:

java.lang.NullPointerException: null
    at clojure.lang.Reflector.invokeNoArgInstanceMember(Reflector.java:296) ~[starman-0.6.5.jar:na]
    at vmfest.virtualbox.host$add_one_host_only_interface.invoke(host.clj:29) ~[na:na]
    at vmfest.virtualbox.host$add_host_only_interface_STAR_.invoke(host.clj:42) ~[na:na]
    at vmfest.virtualbox.host$add_host_only_interface.invoke(host.clj:132) ~[na:na]