Document HttpError

Also type the status.

src/common/http.ts

@@ -8,8 +8,12 @@ export enum HttpCode {
   ServerError = 500,
 }
 
+/**
+ * Represents an error with a message and an HTTP status code. This code will be
+ * used in the HTTP response.
+ */
 export class HttpError extends Error {
-  public constructor(message: string, public readonly status: number, public readonly details?: object) {
+  public constructor(message: string, public readonly status: HttpCode, public readonly details?: object) {
     super(message)
     this.name = this.constructor.name
   }