Provide support for "self describing" Error conforming types

[nbrooke]

Right now all logging of error types must take the form clog.error(message, error), requiring providing a message. But for strongly typed errors, the "message" may just be a restatement of the "type" of the error to some degree, i.e.

func operationFailed(_ error: NetworkError) {
    clog.error("Network error", error)
}

func otherOperationFailed(_ error: SpecificError) {
    switch error {
      case .a : clog.error("failure A", error)
      case .b : clog.error("failure B", error)
    }
}

On one project, we did add direct support for this sort of thing at the app level by adding a type that provides a description, something like:

protocol LoggableError: Error {
   var typeDescription: StaticString { get } 
}

which would allow adding a

func error(_ selfDescribingError: LoggableError) {
  error(selfDescribingError.typeDescription, selfDescribingError)
}

that doesn't need a message. It might be useful to support this at the Steamclog level.