block: Fix block-set-write-threshold not to use funky error class

Error classes are a leftover from the days of "rich" error objects.
New code should always use ERROR_CLASS_GENERIC_ERROR.  Commit e246211
added a use of ERROR_CLASS_DEVICE_NOT_FOUND.  Replace it.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block/write-threshold.c

@@ -112,7 +112,7 @@ void qmp_block_set_write_threshold(const char *node_name,
 
     bs = bdrv_find_node(node_name);
     if (!bs) {
-        error_set(errp, QERR_DEVICE_NOT_FOUND, node_name);
+        error_setg(errp, "Device '%s' not found", node_name);
         return;
     }
 

qapi/block-core.json

@@ -1961,10 +1961,6 @@
 # @write-threshold: configured threshold for the block device, bytes.
 #                   Use 0 to disable the threshold.
 #
-# Returns: Nothing on success
-#          If @node name is not found on the block device graph,
-#          DeviceNotFound
-#
 # Since: 2.3
 ##
 { 'command': 'block-set-write-threshold',