[#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/engine/engine.go

@@ -140,7 +140,7 @@ func (e *StorageEngine) reportShardError(
 	if isLogical(err) {
 		e.log.Warn(ctx, msg,
 			zap.Stringer("shard_id", sh.ID()),
-			zap.String("error", err.Error()))
+			zap.Error(err))
 		return
 	}
 
@@ -151,7 +151,7 @@ func (e *StorageEngine) reportShardError(
 	e.log.Warn(ctx, msg, append([]zap.Field{
 		zap.Stringer("shard_id", sid),
 		zap.Uint32("error count", errCount),
-		zap.String("error", err.Error()),
+		zap.Error(err),
 	}, fields...)...)
 
 	if e.errorsThreshold == 0 || errCount < e.errorsThreshold {