[#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/control.go

@@ -74,7 +74,7 @@ func (b *BlobStor) Close(ctx context.Context) error {
 	for i := range b.storage {
 		err := b.storage[i].Storage.Close(ctx)
 		if err != nil {
-			b.log.Info(ctx, logs.BlobstorCouldntCloseStorage, zap.String("error", err.Error()))
+			b.log.Info(ctx, logs.BlobstorCouldntCloseStorage, zap.Error(err))
 			if firstErr == nil {
 				firstErr = err
 			}