[NuGet] Fix incorrect error message when retargeting project packages.

Error message mentioned updating when it should have mentioned
retargeting. This error message is only displayed if something failed
during the steps before the retargeting was started.
[NuGet] Show packages up to date message in status bar.

When the user updates a single package, all packages in the project or
all packages in the solution and all packages are already up to date
then the status bar now displays a message indicating that the package
or packages are up to date.

Previously the NuGet addin would display a message that the package or
packages were successfully updated when nothing actually happened.