[#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/blobovniczatree/manager.go

@@ -97,7 +97,7 @@ func (b *sharedDB) Close(ctx context.Context) {
 		if err := b.blcza.Close(ctx); err != nil {
 			b.log.Error(ctx, logs.BlobovniczatreeCouldNotCloseBlobovnicza,
 				zap.String("id", b.path),
-				zap.String("error", err.Error()),
+				zap.Error(err),
 			)
 		}
 		b.blcza = nil
@@ -125,7 +125,7 @@ func (b *sharedDB) CloseAndRemoveFile(ctx context.Context) error {
 	if err := b.blcza.Close(ctx); err != nil {
 		b.log.Error(ctx, logs.BlobovniczatreeCouldNotCloseBlobovnicza,
 			zap.String("id", b.path),
-			zap.String("error", err.Error()),
+			zap.Error(err),
 		)
 		return fmt.Errorf("failed to close blobovnicza (path = %s): %w", b.path, err)
 	}