windows: updates: make UI represent underlying update states

This commit fixes several issues:
- the update confirmation dialog getting stuck in the open state
- secondary attempts to download an update possibly causing a crash
- poor error information presented in the case of missing content
- unhandled errors during MSI package installation

Gitlab: #1367
Change-Id: Ia8855b8268ab13b8e1cbbb15de75d41f593f947a