Fix a segfault when we don't correctly load a VFS module (don't keep it in

Fix a segfault when we don't correctly load a VFS module (don't keep it in
the loaded list on error).  Also change some of the error returns, becouse
NT_STATUS_UNSUCCESSFUL gives a most useless error message on the client.

As for which error, my logic is that a share without a valid VFS module is
not a valid share, and therefore should return the same error as a non-existant
share.

Andrew Bartlett