Multi-file: Runtime error spans are incorrectly mapped to source files

[minestarks]

The bug exists in run() in the wasm layer. When the runtime error contains a span, and the span is coming from a file in the program that's not the first one, the wrong span is reported.

I don't think this span is used in the UX anywhere but since it's such a general function it may bite us in the future if we decide to surface these spans.

[sezna]

I need to see if this is still happening after all of the multi-file changes. Assigning myself.