[#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/container/process_container.go

@@ -50,7 +50,7 @@ func (cp *Processor) processContainerPut(ctx context.Context, put putEvent) bool
 	err := cp.checkPutContainer(pctx)
 	if err != nil {
 		cp.log.Error(ctx, logs.ContainerPutContainerCheckFailed,
-			zap.String("error", err.Error()),
+			zap.Error(err),
 		)
 
 		return false
@@ -58,7 +58,7 @@ func (cp *Processor) processContainerPut(ctx context.Context, put putEvent) bool
 
 	if err := cp.morphClient.NotarySignAndInvokeTX(pctx.e.NotaryRequest().MainTransaction); err != nil {
 		cp.log.Error(ctx, logs.ContainerCouldNotApprovePutContainer,
-			zap.String("error", err.Error()),
+			zap.Error(err),
 		)
 		return false
 	}
@@ -113,7 +113,7 @@ func (cp *Processor) processContainerDelete(ctx context.Context, e containerEven
 	err := cp.checkDeleteContainer(e)
 	if err != nil {
 		cp.log.Error(ctx, logs.ContainerDeleteContainerCheckFailed,
-			zap.String("error", err.Error()),
+			zap.Error(err),
 		)
 
 		return false
@@ -121,7 +121,7 @@ func (cp *Processor) processContainerDelete(ctx context.Context, e containerEven
 
 	if err := cp.morphClient.NotarySignAndInvokeTX(e.NotaryRequest().MainTransaction); err != nil {
 		cp.log.Error(ctx, logs.ContainerCouldNotApproveDeleteContainer,
-			zap.String("error", err.Error()),
+			zap.Error(err),
 		)
 
 		return false