blockdev: Clean up fragile use of error_is_set()

Using error_is_set(ERRP) to find out whether a function failed is
either wrong, fragile, or unnecessarily opaque.  It's wrong when ERRP
may be null, because errors go undetected when it is.  It's fragile
when proving ERRP non-null involves a non-local argument.  Else, it's
unnecessarily opaque (see commit 84d18f06).

The error_is_set(errp) in internal_snapshot_prepare() is merely
fragile, because the caller never passes a null errp argument.

Make the code more robust and more obviously correct: receive the
error in a local variable, then propagate it through the parameter.
Signed-off-by: NMarkus Armbruster <armbru@redhat.com>
Reviewed-by: NKevin Wolf <kwolf@redhat.com>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>

blockdev.c

@@ -1115,6 +1115,7 @@ typedef struct InternalSnapshotState {
 static void internal_snapshot_prepare(BlkTransactionState *common,
                                       Error **errp)
 {
+    Error *local_err = NULL;
     const char *device;
     const char *name;
     BlockDriverState *bs;
@@ -1163,8 +1164,10 @@ static void internal_snapshot_prepare(BlkTransactionState *common,
     }
 
     /* check whether a snapshot with name exist */
-    ret = bdrv_snapshot_find_by_id_and_name(bs, NULL, name, &old_sn, errp);
-    if (error_is_set(errp)) {
+    ret = bdrv_snapshot_find_by_id_and_name(bs, NULL, name, &old_sn,
+                                            &local_err);
+    if (local_err) {
+        error_propagate(errp, local_err);
         return;
     } else if (ret) {
         error_setg(errp,