Improves the URLs reported in errors from the host (dotnet/core-setup#5030)

Improves the URLs reported in errors from the host (dotnet/core-setup#5030)

Fixed the URL used when SDK commands fail to find SDK - this should point to the main download page.
Errors which talk about installing prerequisites now include platform specific URLs which point to the respective docs article for the correct platform.

The existing fwlink is now redirected to point to the Windows prereq page. Two new fwlinks were added to point to OSX and Linux pages.

Added simplistic tests for these changes.

Commit migrated from https://github.com/dotnet/core-setup/commit/e6d577688072f3271ea16f6e472c7fad475e6d83