Add InvalidParameter runtime error and return inaccessible for 5xx errors (#1890)

* InvalidParameter: New error type
* Return inaccessible for 5xx errors from services
* Add test for Inaccessible on 5xx
* Add tests for named error types

lib/error-helper.js

@@ -1,6 +1,10 @@
 'use strict'
 
-const { NotFound, InvalidResponse } = require('../services/errors')
+const {
+  NotFound,
+  InvalidResponse,
+  Inaccessible,
+} = require('../services/errors')
 
 const checkErrorResponse = function(
   badgeData,
@@ -27,13 +31,15 @@ const checkErrorResponse = function(
 
 checkErrorResponse.asPromise = function({ notFoundMessage } = {}) {
   return async function({ buffer, res }) {
+    const underlyingError = Error(
+      `Got status code ${res.statusCode} (expected 200)`
+    )
     if (res.statusCode === 404) {
       throw new NotFound({ prettyMessage: notFoundMessage })
+    } else if (res.statusCode >= 500) {
+      throw new Inaccessible({ underlyingError })
     } else if (res.statusCode !== 200) {
-      const underlying = Error(
-        `Got status code ${res.statusCode} (expected 200)`
-      )
-      throw new InvalidResponse({ underlyingError: underlying })
+      throw new InvalidResponse({ underlyingError })
     }
     return { buffer, res }
   }