Currently VolOpen notifies the user of a potentially non-fatal failure by
returning -2 and logging a VIR_WARN or VIR_INFO. Unfortunately most
callers treat -2 as fatal but don't actually report any message with
the error APIs.

Rename the VOL_OPEN_ERROR flag to VOL_OPEN_NOERROR. If NOERROR is specified,
we preserve the current behavior of returning -2 (there's only one caller
that wants this).

However in the default case, only return -1, and actually use the error
APIs. Fix up a couple callers as a result.

(cherry picked from commit 138e65c3)

Conflicts:
	src/storage/storage_backend.c
	src/storage/storage_backend_fs.c