[appveyor] Error handling in BaseService (#1590)

Make a clear distinction between programmer errors ("internal errors") and runtime errors, and allow configuring the server to let the programmer errors bubble up in development and unit testing. This saves a huge amount of time because it generates ordinary stack traces when things go wrong. And, if these errors occur in production, we'll catch them, and display **shields | internal error** which is the equivalent of a 500 error.

lib/error-helper.js

@@ -1,5 +1,10 @@
 'use strict';
 
+const {
+  NotFound,
+  InvalidResponse,
+} = require('../services/errors');
+
 const checkErrorResponse = function(badgeData, err, res, notFoundMessage = 'not found') {
   if (err != null) {
     badgeData.text[1] = 'inaccessible';
@@ -18,6 +23,27 @@ const checkErrorResponse = function(badgeData, err, res, notFoundMessage = 'not
   }
 };
 
+checkErrorResponse.asPromise = function ({ notFoundMessage } = {}) {
+  return async function ({ buffer, res }) {
+    if (res.statusCode === 404) {
+      throw new NotFound(notFoundMessage);
+    } else if (res.statusCode !== 200) {
+      const underlying = Error(`Got status code ${res.statusCode} (expected 200)`);
+      throw new InvalidResponse(undefined, underlying);
+    }
+    return { buffer, res };
+  };
+};
+
+async function asJson({ buffer, res }) {
+  try {
+    return JSON.parse(buffer);
+  } catch (err) {
+    throw new InvalidResponse();
+  }
+};
+
 module.exports = {
   checkErrorResponse,
+  asJson,
 };