Issue title: HttpRequestException when running offline
 Issue body: When running offline (which is always our use-case) an HttpRequestException occurs and displays on the main page.

Version 2.1.133+ddb0c80733
 Comments: 
 Comment 0: Yes, the app is trying to update itself, I can tone it down.
 Comment 1: I've made a better error message that is not as obtrusive.

You can also turn off automatic updates entirely if you want by creating an (empty) .txt file:
`%localappdata%\Microsoft\MSBuildStructuredLog\DisableUpdates.txt`

If this file is present the app will not attempt to update.
{'source': 'GithubIssues', 'repo_name_with_owner': 'KirillOsenkov/MSBuildStructuredLog', 'issue_id': 621077556, 'repo_stars': 1202.0}