[#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/morph/client/client.go

@@ -390,7 +390,7 @@ func (c *Client) Wait(ctx context.Context, n uint32) error {
 	height, err = c.rpcActor.GetBlockCount()
 	if err != nil {
 		c.logger.Error(ctx, logs.ClientCantGetBlockchainHeight,
-			zap.String("error", err.Error()))
+			zap.Error(err))
 		return nil
 	}
 
@@ -404,7 +404,7 @@ func (c *Client) Wait(ctx context.Context, n uint32) error {
 		newHeight, err = c.rpcActor.GetBlockCount()
 		if err != nil {
 			c.logger.Error(ctx, logs.ClientCantGetBlockchainHeight243,
-				zap.String("error", err.Error()))
+				zap.Error(err))
 			return nil
 		}
 

pkg/morph/event/listener.go

@@ -136,7 +136,7 @@ func (l *listener) Listen(ctx context.Context) {
 		defer l.wg.Done()
 		if err := l.listen(ctx, nil); err != nil {
 			l.log.Error(ctx, logs.EventCouldNotStartListenToEvents,
-				zap.String("error", err.Error()),
+				zap.Error(err),
 			)
 		}
 	})
@@ -154,7 +154,7 @@ func (l *listener) ListenWithError(ctx context.Context, intError chan<- error) {
 		defer l.wg.Done()
 		if err := l.listen(ctx, intError); err != nil {
 			l.log.Error(ctx, logs.EventCouldNotStartListenToEvents,
-				zap.String("error", err.Error()),
+				zap.Error(err),
 			)
 			l.sendError(ctx, intError, err)
 		}
@@ -342,7 +342,7 @@ func (l *listener) parseAndHandleNotification(ctx context.Context, notifyEvent *
 	event, err := parser(notifyEvent)
 	if err != nil {
 		log.Warn(ctx, logs.EventCouldNotParseNotificationEvent,
-			zap.String("error", err.Error()),
+			zap.Error(err),
 		)
 
 		return
@@ -375,13 +375,13 @@ func (l *listener) parseAndHandleNotary(ctx context.Context, nr *result.NotaryRe
 		case errors.Is(err, ErrTXAlreadyHandled):
 		case errors.As(err, &expErr):
 			l.log.Warn(ctx, logs.EventSkipExpiredMainTXNotaryEvent,
-				zap.String("error", err.Error()),
+				zap.Error(err),
 				zap.Uint32("current_block_height", expErr.CurrentBlockHeight),
 				zap.Uint32("fallback_tx_not_valid_before_height", expErr.FallbackTXNotValidBeforeHeight),
 			)
 		default:
 			l.log.Warn(ctx, logs.EventCouldNotPrepareAndValidateNotaryEvent,
-				zap.String("error", err.Error()),
+				zap.Error(err),
 			)
 		}
 
@@ -413,7 +413,7 @@ func (l *listener) parseAndHandleNotary(ctx context.Context, nr *result.NotaryRe
 	event, err := parser(notaryEvent)
 	if err != nil {
 		log.Warn(ctx, logs.EventCouldNotParseNotaryEvent,
-			zap.String("error", err.Error()),
+			zap.Error(err),
 		)
 
 		return

pkg/morph/event/utils.go

@@ -77,7 +77,7 @@ func WorkerPoolHandler(w util2.WorkerPool, h Handler, log *logger.Logger) Handle
 		})
 		if err != nil {
 			log.Warn(ctx, logs.EventCouldNotSubmitHandlerToWorkerPool,
-				zap.String("error", err.Error()),
+				zap.Error(err),
 			)
 		}
 	}