[#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/innerring.go

@@ -177,7 +177,7 @@ func (s *Server) Start(ctx context.Context, intError chan<- error) (err error) {
 	if err != nil {
 		// we don't stop inner ring execution on this error
 		s.log.Warn(ctx, logs.InnerringCantVoteForPreparedValidators,
-			zap.String("error", err.Error()))
+			zap.Error(err))
 	}
 
 	s.tickInitialExpoch(ctx)
@@ -308,7 +308,7 @@ func (s *Server) Stop(ctx context.Context) {
 	for _, c := range s.closers {
 		if err := c(); err != nil {
 			s.log.Warn(ctx, logs.InnerringCloserError,
-				zap.String("error", err.Error()),
+				zap.Error(err),
 			)
 		}
 	}