Add asserts to narrow down position issue

[uniqueiniquity]

In Visual Studio, we're seeing hundreds of hits where a location returned in semanticDiagnosticsSync has either a non-positive line or non-positive offset.

To narrow down the investigation, I'm adding these asserts so that we have just a little more context to help the investigation (that is, I think it would be helpful to know whether the bad value is null/undefined, 0, or negative).

[armanio123]

Looks good to me. I wonder if it's a good idea to do some work to include the server request and response on the telemetry-bot as additional information. We can discuss this offline.

[uniqueiniquity]

I've added a couple more asserts to handle the input - for example, I think that if we get undefined as input, we might output NaN.