Add an error message if the wrong platform-specific VSIX is installed

[Colengms]

Prior to switching to platform-specific VSIX's via the marketplace, we would display an error message if we detected that the wrong platform binaries were installed (if the user had manually downloaded and installed the wrong platform VSIX). This error message should be added back in. There is still the potential to download the wrong platform-specific VSIX from the marketplace web site. There is also an issue in which VS Code may sometimes install the wrong VSIX: microsoft/vscode#141696

There is a TargetPlatform field written to the .vsixmanifest file at the root of the extension directory (when installed from a VSIX), that could be used for this check.