Display warning on attempt to uninstall chocolatey package

The following message is output when running choco uninstall choco:
"choco is not installed. Cannot uninstall a non-existent package."

It would be great if choco considered itself an installed package and could uninstall itself.

At the very least it would be more intuitive to say something to the effect of
"choco cannot uninstall itself" or "choco is the package manager, not a package. To uninstall choco please do the following..."

Start by reproducing `choco uninstall choco` and tracing the uninstall command's handling of the `choco` package name. Done means the command no longer reports that choco is a nonexistent package, but instead displays a clear warning or instructions explaining that the package manager cannot uninstall itself.

Written by the indexing model from the issue text.