Caching: investigate catching shared folder failure and providing useful message

[cheeseplus]

If Vagrant barfs on loading the shared folders and we can reliably catch that message for the providers then we should print a nice message showing how to disable shared folders.

Even if they are using bento boxes there might be hypervisor vs vmtools incompatibilities that prevent the shared folder from mounting so this would at least provide a clear path forward for users.

[cheeseplus]

Although I need to push the box to Vagrant Cloud, Debian 9.x vmware boxes currently break with shared folders and as such are a great test case.