Created Issue: Install from packages.config silently skips missing/invalid packages [2819]

Steps to reproduce:
1 - Create a packages.config file with a packageId and version that doesn't exist in a given shared source, such as "Invalid" and "99.0.0.0" (this can happen if a developer tested a package from a local folder on their computer but didn't publish it to said shared source before checking in the updated packages.config file).
2 - `nuget install path/to/packages.config -source "http://sharedSource/NuGet/nuget" -o "packages"` (i.e. the "RestoreCommand" in NuGet.targets)

Expected results:
1 - "Successfully installed <packageId> <version>" for each package that was found.
2 - "Unable to find version '99.0.0.0' of package 'Invalid'.
3 - NuGet.exe returns an error code.

Actual results:
1 - "Successfully installed <packageId> <version>" for each package that was found.
2 - ???
3 - When launched as part of package restore, compilation continues anyway.

Notes:
1 - Running the following:
`nuget install Invalid -Version 99.0.0.0 -source "http://sharedSource/NuGet/nuget"`
...actually produces the desired result, so we just need it to fail when running from a packages.config file.