[NuGet] Different message when no updates found but warnings logged.

When one of the package sources is unavailable or invalid then NuGet
will report a warning when checking for updates. Previously the NuGet
addin would display the following message even though no packages
were updated:

"Packages updated with warnings. Please see Package Console for
details."

One way to reproduce this is to have the main NuGet.org feed enabled
and add another NuGet package source with a url that does not have
a package source or a host name that cannot be resolved. NuGet will
find no package updates at the main NuGet.org package source and
report warnings for the other package sources.

Now the NuGet addin has been modified so that when no updates were
found but warnings were reported the status bar message now reports
the following message:

"No updates found but warnings were reported. Please see Package
Console for details."

Longer term the NuGet addin should determine if the warning reported
was due to a networking problem and then display a more specific error
message should be displayed. Currently this is not implemented.