VFD should fall back to POSIX I/O when Hermes functions fail

[ChristopherHogan]

Currently the return status of Put is ignored in the VFD. We should do the same thing we do for other adapters, which is to fall back to POSIX when Hermes fails to buffer data. See hermes/adapter/posix/posix.cc:PutWithPosixFallback for an example.