runWarnings format difficult for pragmatic usage

I have two concerns with `runWarnings` specifically around pragmatic usage:

* `runWarnings` contains both errors (eg, page hung / non-successful request on main document resource) and warnings (eg, CPU throttling might be bad)
* The errors and warnings contained within `runWarnings` are strings. There's no handle to be able to identify one warning/error from another, without searching the string.

**Feature request summary**

`runWarnings` to be changed to an object. It should contain `code` and `message`. Just like `runtimeError`.

**What is the motivation or use case for changing this?**

Node.js/API/PageSpeed users of Lighthouse will be able to handle errors/warnings reliably.