Improve Symbol Uploader error reporting

[JohnTortugo]

Please see context here: dotnet/arcade#4304 (comment)

From @mikem8361 :

The NullReferenceException definitely needs to be fixed but I'm sure it is either network time out or error path. I can make some simple fixes to log all inner exceptions in the aggregated one to diagnose this in the future.

Example builds:
https://dnceng.visualstudio.com/internal/_build/results?buildId=419023
https://dnceng.visualstudio.com/internal/_build/results?buildId=417308

[mikem8361]

I have built a new version of the symbol uploader with better failure diagnostics. The null ref hit isn't fixed but the changes I made should help find it if it happens again.

New version 1.0.0-beta-64708-02. It is still being pushed to the buildtools myget feed. I still have some engineering to do to publish it to the dotnet-core blob feed. It is also on \bvtsrv2\team\DotNetCore\DotNet-SymUploader\20191108.02\packages\Shipping