blockdev: Fix block_resize error reporting for op blockers

When block_resize() runs into an op blocker, it creates an error like
this:

        error_setg(errp, "Device '%s' is in use", device);

Trouble is @device can be null.  My system formats null as "(null)",
but other systems might crash.  Reproducer:

1. Create two block devices

    -> {"execute": "blockdev-add", "arguments": {"driver": "file", "node-name": "blk0", "filename": "64k.img"}}
    <- {"return": {}}
    -> {"execute": "blockdev-add", "arguments": {"driver": "file", "node-name": "blk1", "filename": "m.img"}}
    <- {"return": {}}

2. Put a blocker on one them

    -> {"execute": "blockdev-mirror", "arguments": {"job-id": "job0", "device": "blk0", "target": "blk1", "sync": "full"}}
    {"return": {}}
    -> {"execute": "job-pause", "arguments": {"id": "job0"}}
    {"return": {}}
    -> {"execute": "job-complete", "arguments": {"id": "job0"}}
    {"return": {}}

   Note: job events elided for brevity.

3. Attempt to resize

    -> {"execute": "block_resize", "arguments": {"node-name": "blk1", "size":32768}}
    <- {"error": {"class": "GenericError", "desc": "Device '(null)' is in use"}}

Broken when commit 3b1dbd11a6 made @device optional.  Fixed in commit
ed3d2ec98a (block: Add errp to b{lk,drv}_truncate()), except for this
one instance.

Fix it by using the error message provided by the op blocker instead,
so it fails like this:

    <- {"error": {"class": "GenericError", "desc": "Node 'blk1' is busy: block device is in use by block job: mirror"}}

Fixes: 3b1dbd11a6 (qmp: Allow block_resize to manipulate bs graph nodes.)
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Michael Tokarev <mjt@tls.msk.ru>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

blockdev.c

@@ -2252,8 +2252,7 @@ void coroutine_fn qmp_block_resize(const char *device, const char *node_name,
     }
 
     bdrv_graph_co_rdlock();
-    if (bdrv_op_is_blocked(bs, BLOCK_OP_TYPE_RESIZE, NULL)) {
-        error_setg(errp, QERR_DEVICE_IN_USE, device);
+    if (bdrv_op_is_blocked(bs, BLOCK_OP_TYPE_RESIZE, errp)) {
         bdrv_graph_co_rdunlock();
         return;
     }