Consider using special type for error reporting

https://cloud.google.com/error-reporting/docs/formatting-error-messages#@type

Mostly we just have to test this.. It might make error reporting work for severity other than `>= ERROR`.

I'm not sure this is desirable, but then we could implement the limit in the `package:logging` adapter.
Currently, errors included in warnings are not reported here. Which I guess has pros/cons.