Don't fail if the user is offline

[nagilson]

Resolves #1866
#1858 (does not add api, we decide to do this automatically)

The VersionResolver class makes a web-request before downloading .NET to find the newest version of .NET to keep users up to date. The problem is, this resolution of which version of .NET to download occurs before when we check the pre-existing installations to see if we need to do an installation at all, because we need to resolve a version string such as '8.0' to '8.0.2'.

If the user had downloaded the .NET before hand then went on an airplane, which many have, it would not check if that install exists because it'd first see if there is a newer version to download then fail.

This adds a check where if we are offline we check the preexisting installs first to prevent this failure from occurring.

Previous Behavior:

It would wait for the timeout silently and then fail.
See users reporting this in microsoft/vscode-dotnettools#1028 which this resolves.

New Behavior:

For the first 2 requests, I request '7.0' which was installed previously. It uses the old existing install and remarks to update when possible.

For the next request, I request '8.0' which was not installed previously. It has a new warning message instead of hanging and will wait..

When connecting back to the internet, it works again.

[smitpatel]

Please split the function to handle command in different files. It is making harder to review changes in extension.ts file as it is hard to find which function is getting modified.

[nagilson]

Thanks for your review, you caught some important issues. For the refactor, I've made a separate issue to track this so it can be done in another PR: #1917