[#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/iterate.go

@@ -45,7 +45,7 @@ func (b *BlobStor) Iterate(ctx context.Context, prm common.IteratePrm) (common.I
 				b.log.Warn(ctx, logs.BlobstorErrorOccurredDuringTheIteration,
 					zap.String("storage_path", b.storage[i].Storage.Path()),
 					zap.String("storage_type", b.storage[i].Storage.Type()),
-					zap.String("err", err.Error()))
+					zap.Error(err))
 				continue
 			}
 			return common.IterateRes{}, fmt.Errorf("blobstor iterator failure: %w", err)