[#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/innerring/processors/frostfs/process_assets.go

@@ -73,7 +73,7 @@ func (np *Processor) processDeposit(ctx context.Context, deposit frostfsEvent.De
 	err = np.morphClient.TransferGas(receiver, np.mintEmitValue)
 	if err != nil {
 		np.log.Error(ctx, logs.FrostFSCantTransferNativeGasToReceiver,
-			zap.String("error", err.Error()))
+			zap.Error(err))
 
 		return false
 	}