show error message when app already running

this adds a new error type when app is not able to
get a lock of the pid file as another instance  is
already running, it then shows a msg and quits

gui/components/InitErrors.qml

@@ -44,6 +44,19 @@ ErrorBox {
                 errorText: qsTr("Could not find polkit agent.")
                 visible: true 
             }
+        },
+        State {
+            name: "alreadyrunning"
+            when: root.error == "alreadyrunning"
+            PropertyChanges {
+                target: splashProgress
+                visible: false
+            }
+            PropertyChanges {
+                target: splashErrorBox
+                errorText: qsTr("Application is going to quit as another instance is already running. Please use the system tray icon to open it")
+                visible: true 
+            }
         }
     ]
 }

pkg/backend/init.go

 package backend
 
 import (
+	"errors"
+	"os"
+	"time"
+
 	"github.com/rs/zerolog/log"
 
 	"0xacab.org/leap/bitmask-vpn/pkg/bitmask"
@@ -61,10 +65,15 @@ func initializeBitmask(errCh chan string, opts *InitOpts) {
 	ctx.UseUDP = ctx.cfg.UDP
 
 	err := pid.AcquirePID()
-	if err != nil {
-		log.Fatal().
+	if err != nil && errors.Is(err, pid.AlreadyRunningErr) {
+		log.Error().
 			Err(err).
 			Msg("Could not acquire PID")
+		errCh <- "alreadyrunning"
+		// would be better to quit the app from cpp land
+		// using go trigger(OnInitPidError)
+		time.Sleep(10 * time.Second)
+		os.Exit(1)
 	}
 
 	b, err := bitmask.InitializeBitmask(ctx.cfg)

pkg/pid/pid.go

@@ -16,6 +16,7 @@
 package pid
 
 import (
+	"errors"
 	"fmt"
 	"io/ioutil"
 	"os"
@@ -30,17 +31,20 @@ import (
 	"github.com/keybase/go-ps"
 )
 
-var pidFile = filepath.Join(config.Path, "systray.pid")
+var (
+	pidFile           = filepath.Join(config.Path, "systray.pid")
+	AlreadyRunningErr = errors.New("another instance is already running")
+)
 
 func AcquirePID() error {
 	pid := syscall.Getpid()
 	current, err := getPID()
 	if err != nil {
-		log.Print("Error reading pid file:", err)
+		log.Err(err).Msg("Error reading pid file")
 	}
 
 	if current != pid && pidRunning(current) {
-		return fmt.Errorf("Another systray is running with pid: %d", current)
+		return fmt.Errorf("%w, pid: %d", AlreadyRunningErr, current)
 	}
 
 	return setPID(pid)