Warn users that Cake.exe is missing when calling "Install Intellisense Support"

When checking out a repository clean, common sense would dictate that after executing the "Install intellisense support" Intellisense would work right off the bat (Given that troubleshooting steps have been taken (like making sure OmniSharp is installed, or that the Project has been loaded).

But what happens is, that for OmniSharp to work it is necessary that the regular bootstrapper has been called beforehand, because otherwise Cake.exe is not available.

Cake-VSCode should either:
- spawn an Error Fly-In prompting the user to the lack of the Cake.Exe
- call the generation of the cake.exe by itself

Start at the Install Intellisense Support command and trace how it behaves when Cake.exe is absent after a clean checkout. Check the existing extension flow and determine whether the expected outcome is an error fly-in or automatic Cake.exe generation. Done means the missing prerequisite is handled explicitly so users are not left with non-working Intellisense.

Written by the indexing model from the issue text.