[#878] node: Drain internal error's channel

This fixes shutdown panic:
1. Some morph connection gets error and passes it to internalErr channel.
2. Storage node starts to shutdow and closes internalErr channel.
3. Other morph connection gets error and tries to pass it to internalErr channel.

Signed-off-by: Dmitrii Stepanov <d.stepanov@yadro.com>

cmd/frostfs-node/config.go

@@ -1328,5 +1328,4 @@ func (c *cfg) shutdown() {
 	for i := range c.closers {
 		c.closers[len(c.closers)-1-i].fn()
 	}
-	close(c.internalErr)
 }

cmd/frostfs-node/main.go

@@ -6,6 +6,7 @@ import (
 	"fmt"
 	"log"
 	"os"
+	"sync"
 
 	"git.frostfs.info/TrueCloudLab/frostfs-node/cmd/frostfs-node/config"
 	"git.frostfs.info/TrueCloudLab/frostfs-node/internal/logs"
@@ -155,9 +156,22 @@ func wait(c *cfg) {
 
 	<-c.done // graceful shutdown
 
+	drain := &sync.WaitGroup{}
+	drain.Add(1)
+	go func() {
+		defer drain.Done()
+		for err := range c.internalErr {
+			c.log.Warn(logs.FrostFSNodeInternalApplicationError,
+				zap.String("message", err.Error()))
+		}
+	}()
+
 	c.log.Debug(logs.FrostFSNodeWaitingForAllProcessesToStop)
 
 	c.wg.Wait()
+
+	close(c.internalErr)
+	drain.Wait()
 }
 
 func (c *cfg) onShutdown(f func()) {