fix ACTION_OPEN_DOCUMENT_TREE error string so to be more informative

app/src/main/res/values/strings.xml

@@ -522,5 +522,5 @@
     <string name="slide_to_cancel">Slide to Cancel</string>
     <string name="disable_screen_transitions">Disable screen transitions</string>
     <string name="invalid_format">Invalid format</string>
-    <string name="no_directory_picker_activity">No activity found to select directory</string>
+    <string name="no_directory_picker_activity">Barinsta cannot launch Android\'s file manager. Please make sure it is installed and enabled on your device.</string>
 </resources>