Improve the error experience when the reading of the runtime.json fails due to a malformed file

[nkolev92]

Right now that will lead to an error like the following:

C:\Program Files\dotnet\sdk\3.0.100-preview5-011568\NuGet.targets(121,5): error : After parsing a value an unexpected character was encountered: ". Path 'alpine-x64-corert.#import', line 24, position 4. [F:\test\7351\7351.csproj]

To repro this:

Create a project, and add runtime identifiers and run restore.
To provide a bad runtime json, either:

1. Redirect to a custom global packages folder and make edits to microsoft.netcore.platforms in a breaking way (you might need to disable the fallback folder otherwise NuGet will never download it).
2. Create your own package with a runtime.json.

Found while working on #7351.

Note that this has been an issue for a very long time and we have heard no reports whatsoever.
Mostly because no one really authors runtime.json that frequently :)

[nkolev92]

Closing as low pri