Improve error message when failing to open a backing file

When bhyve cannot open a backing file, it now says explicitly which file
could not be opened

Note that the change has only be maed in block_if.c and not in
pci_virtio_block.c as the error will always be catched by the first

PR: 202321 (different patch)
Reviewed by: grehan
MFC after: 3 day
Sponsored by: Gandi.net
Differential Revision: https://reviews.freebsd.org/D6576