pnfs/blocklayout: improve GETDEVICEINFO error reporting

Tell userspace what stage of GETDEVICEINFO failed so that there is a chance
to debug it, especially with the userspace daemon clusterf***k in the block
layout driver.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>