[#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/services/policer/process.go

@@ -61,7 +61,7 @@ func (p *Policer) shardPolicyWorker(ctx context.Context) {
 						if err != nil && !skipMap.addSeenError(addr.Address.Container(), err) {
 							p.log.Error(ctx, logs.PolicerUnableToProcessObj,
 								zap.Stringer("object", addr.Address),
-								zap.String("error", err.Error()))
+								zap.Error(err))
 						}
 						p.cache.Add(addr.Address, time.Now())
 						p.objsInWork.remove(addr.Address)