Added inner exception to error in case of MsBuild solution file parse failure

[alexangas]

Fixes NuGet/Home#4411

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[rohit21agrawal]

@ArangaS thanks for the PR! This has been planned for the next milestone, so we will merge this in as soon as the work on that begins.

[DaveInCaz]

This was mentioned here: https://stackoverflow.com/questions/45100594/nuget-restore-exception-has-been-thrown-by-the-target-of-an-invocation