Improve resolver failure messages

[emgarten]

Resolver constraint error messages do not show enough information in some cases.

Steps

1. Create a packages.config with an invalid combination of packages which violate the constraints
2. Update a package that will bring in new dependencies but still fail

Expected:
If the original constraint problem is the core issue, that will be surfaced in the message

Actual:
NuGet favors messages around new dependencies since they are usually the problem, but in this case the existing packages are the problem and they are not listed in the error message.

[emgarten]

This is only for packages.config, not xplat scenarios.

My current thinking is that this should be fixed by using a simple failure message from the optimistic resolver once that perf improvement goes in.

[rrelyea]

Please port to 3.4.5.

[jainaashish]

Merged with 5c85723fb07d28a7a305d93869e23013d72a2bd0