[#1502] Use zap.Error for logging errors

Use `zap.Error` instead of `zap.String` for logging errors: change all expressions like
`zap.String("error", err.Error())` or `zap.String("err", err.Error())` to `zap.Error(err)`.
Leave similar expressions with other messages unchanged, for example,
`zap.String("last_error", lastErr.Error())` or `zap.String("reason", ctx.Err().Error())`.

This change was made by applying the following patch:
```diff
@@
var err expression
@@
-zap.String("error", err.Error())
+zap.Error(err)

@@
var err expression
@@
-zap.String("err", err.Error())
+zap.Error(err)
```

Signed-off-by: Aleksey Savchuk <a.savchuk@yadro.com>

pkg/local_object_storage/blobstor/fstree/fstree.go

@@ -153,7 +153,7 @@ func (t *FSTree) iterate(ctx context.Context, depth uint64, curPath []string, pr
 	if err != nil {
 		if prm.IgnoreErrors {
 			t.log.Warn(ctx, logs.BlobstorErrorOccurredDuringTheIteration,
-				zap.String("err", err.Error()),
+				zap.Error(err),
 				zap.String("directory_path", dirPath))
 			return nil
 		}
@@ -202,7 +202,7 @@ func (t *FSTree) iterate(ctx context.Context, depth uint64, curPath []string, pr
 			if prm.IgnoreErrors {
 				t.log.Warn(ctx, logs.BlobstorErrorOccurredDuringTheIteration,
 					zap.Stringer("address", addr),
-					zap.String("err", err.Error()),
+					zap.Error(err),
 					zap.String("path", path))
 				continue
 			}