feat(windows): notification when "Download keyboard..." while offline

**Is your feature request related to a problem? Please describe.**


There is no message telling the user what is going on why they are trying to "Download keyboard..." while offline. A plain white screen is shown.

**Describe the solution you'd like**

It may be helpful to add a message telling the user that they are offline and advise them to go online to be able to download keyboard. It is very obvious, but it is a good thing to have a message notifying them the issue they are running into and how to go about it.

**Describe alternatives you've considered**

A popup message would do as well, if not a web page notifying them what is going on.

---

**Keyman for Windows:**
- OS: Windows 10 20H2 x64
- Keyman version: 14.0.267-beta
- Target Application: Keyman