agent: Fix assignment of error when auto-reloading cert and key file changes. (#15769)

* Adding the setting of errors missing in config file watcher code in agent.

* add changelog

.changelog/15769.txt

@@ -0,0 +1,3 @@
+```release-note:bug
+agent: Fix assignment of error when auto-reloading cert and key file changes.
+```

agent/agent.go

@@ -5,7 +5,6 @@ import (
 	"crypto/tls"
 	"encoding/json"
 	"fmt"
-	"github.com/hashicorp/consul/proto/pboperator"
 	"io"
 	"net"
 	"net/http"
@@ -18,6 +17,8 @@ import (
 	"sync"
 	"time"
 
+	"github.com/hashicorp/consul/proto/pboperator"
+
 	"github.com/armon/go-metrics"
 	"github.com/armon/go-metrics/prometheus"
 	"github.com/hashicorp/go-connlimit"
@@ -4026,13 +4027,13 @@ func (a *Agent) reloadConfig(autoReload bool) error {
 			{a.config.TLS.HTTPS, newCfg.TLS.HTTPS},
 		} {
 			if f.oldCfg.KeyFile != f.newCfg.KeyFile {
-				a.configFileWatcher.Replace(f.oldCfg.KeyFile, f.newCfg.KeyFile)
+				err = a.configFileWatcher.Replace(f.oldCfg.KeyFile, f.newCfg.KeyFile)
 				if err != nil {
 					return err
 				}
 			}
 			if f.oldCfg.CertFile != f.newCfg.CertFile {
-				a.configFileWatcher.Replace(f.oldCfg.CertFile, f.newCfg.CertFile)
+				err = a.configFileWatcher.Replace(f.oldCfg.CertFile, f.newCfg.CertFile)
 				if err != nil {
 					return err
 				}