Use generic typed error pattern

The codebase doesn't yet use the explicitly typed error pattern. To keep
things consistent, lets use the generic error type.

pkg/bookmarks/bookmarks.go

@@ -90,20 +90,14 @@ func ReadAll(path string) (map[string]Bookmark, error) {
 	return results, nil
 }
 
-type ErrNonExistingBookmark string
-
-func (e ErrNonExistingBookmark) Error() string {
-	return fmt.Sprintf("couldn't find a bookmark with name %s", e)
-}
-
 func GetBookmark(bookmarkPath string, bookmarkName string) (Bookmark, error) {
 	bookmarks, err := ReadAll(bookmarkPath)
 	if err != nil {
 		return Bookmark{}, err
 	}
 	bookmark, ok := bookmarks[bookmarkName]
 	if !ok {
-		return Bookmark{}, ErrNonExistingBookmark(bookmarkName)
+		return Bookmark{}, fmt.Errorf("couldn't find a bookmark with name %s", bookmarkName)
 	}
 	return bookmark, nil
 

pkg/bookmarks/bookmarks_test.go

@@ -84,7 +84,8 @@ func Test_GetBookmark(t *testing.T) {
 	}
 
 	_, err = GetBookmark("../../data", "bar")
-	assert.Equal(t, ErrNonExistingBookmark("bar"), err)
+	expErrStr := "couldn't find a bookmark with name bar"
+	assert.Equal(t, expErrStr, err.Error())
 
 	_, err = GetBookmark("foo", "bookmark")
 	assert.Error(t, err)