add error when matchedFolders is empty

folderwatcher.go

@@ -1,6 +1,7 @@
 package folderwatcher
 
 import (
+	"fmt"
 	"github.com/sirupsen/logrus"
 	"os"
 	"path/filepath"
@@ -52,7 +53,18 @@ func NewFolderWatcher(conf Config, includeExisting bool, quit chan struct{}) (*F
 		}
 	}
 
+	if len(matchedFolders) == 0 {
+		return nil, fmt.Errorf("no folders found matching '%s'", conf.Folder)
+	}
+
 	for _, path := range matchedFolders {
+		stat, err := os.Stat(path)
+		if err != nil {
+			return nil, err
+		}
+		if !stat.IsDir() {
+			return nil, fmt.Errorf("%s is not a folder", path)
+		}
 		err = watcher.Add(path)
 		if err != nil {
 			return nil, err

folderwatcher_notExistingFolder_test.go

@@ -0,0 +1,22 @@
+package folderwatcher
+
+import (
+	"os"
+	"testing"
+)
+
+func TestNotExistingFolder(t *testing.T) {
+	tmpPath, err := os.MkdirTemp("", ".folderwatcher_test")
+	if err != nil {
+		t.Fatal(err)
+	}
+	_ = os.RemoveAll(tmpPath)
+	conf := Config{
+		Folder: tmpPath,
+	}
+
+	_, err = NewFolderWatcher(conf, true, nil)
+	if err == nil {
+		t.Fatal("expected error")
+	}
+}