cmd/wails: report error status code to OS (#252)

Fixes #251.

cmd/wails/main.go

@@ -1,6 +1,9 @@
 package main
 
 import (
+	"os"
+	"os/exec"
+
 	"github.com/wailsapp/wails/cmd"
 )
 
@@ -15,5 +18,9 @@ func main() {
 	err := app.Run()
 	if err != nil {
 		logger.Error(err.Error())
+		if exitErr, ok := err.(*exec.ExitError); ok {
+			os.Exit(exitErr.ExitCode())
+		}
+		os.Exit(1)
 	}
 }