Package apidiags provides types to construct API-level Diagnostics,
information that can be returned with a response to specify errors or warnings.
They are shamelessly "inspired" by Martin Atkins' work on
zcl and the Diagnostics introduced there.
A Diagnostic indicates some information about an API request that should be returned to the caller. It has a Severity (indicating whether it should be treated as an error or warning), a Code (indicating the information being communicated), and Paths (indicating the part(s) of the request that triggered the Diagnostic). Each time a Diagnostic would be triggered, even if it uses the same Code, should result in a new Diagnostic being added to the request rather than adding another Path to the Diagnostic. The ability to specify multiple Paths in a single Diagnostic is intended to allow Diagnostics to describe a conflict between two parts of a request, or other similar situations where a single part of a request is insufficient for indicating what went wrong.