Improve error message for missing apiKey and apiSecret

I was debugging an error recently where I saw the following logged:

> No `apiKey` or `apiSecret` found in config. Falling back to
> pull-request authentication.

I believe in this particular case that only one of these was missing and
I thought it might be nice if this error message was a little more
specifically helpful.

src/loadUserConfig.js

@@ -60,37 +60,47 @@ export default async function loadUserConfig(pathToConfigFile, env = process.env
 
   const config = await load(pathToConfigFile);
   if (!config.apiKey || !config.apiSecret) {
+    const missing = [
+      !config.apiKey ? 'apiKey' : null,
+      !config.apiSecret ? 'apiSecret' : null,
+    ]
+      .filter(Boolean)
+      .map((key) => `\`${key}\``)
+      .join(' and ');
+
     const prUrl = CHANGE_URL || resolvePRLink(env);
     if (!prUrl) {
       throw new Error(
-        'You need an `apiKey` and `apiSecret` in your config. ' +
-          'To obtain one, go to https://happo.io/settings',
+        `Missing ${missing} in your Happo config. Reference yours at https://happo.io/settings`,
       );
     }
+
     try {
-      // Reassign api tokens to temporary ones provided for the PR
+      // Reassign API tokens to temporary ones provided for the PR
       new Logger().info(
-        'No `apiKey` or `apiSecret` found in config. Falling back to pull-request authentication.',
+        `Missing ${missing} in Happo config. Falling back to pull-request authentication.`,
       );
       config.apiKey = prUrl;
       config.apiSecret = await getPullRequestSecret(config, prUrl);
     } catch (e) {
       throw new WrappedError('Failed to obtain temporary pull-request token', e);
     }
   }
+
   if (!config.targets || Object.keys(config.targets).length === 0) {
     throw new Error(
-      'You need at least one target defined under `targets`. ' +
-        'See https://github.com/happo/happo.io#targets for more info.',
+      'You need at least one target defined under `targets` in your Happo config. See https://github.com/happo/happo.io#targets for more info.',
     );
   }
+
   const defaultKeys = Object.keys(defaultConfig);
   const usedKeys = Object.keys(config);
   usedKeys.forEach((key) => {
     if (!defaultKeys.includes(key)) {
       new Logger().warn(`Unknown config key used in .happo.js: "${key}"`);
     }
   });
+
   config.publicFolders.push(config.tmpdir);
   config.plugins.forEach(({ publicFolders }) => {
     if (publicFolders) {

test/loadUserConfig-test.js

@@ -26,9 +26,25 @@ beforeEach(() => {
   }));
 });
 
-it('yells if api tokens are missing', async () => {
+it('yells if both API tokens are missing', async () => {
   requireRelative.mockImplementation(() => ({}));
-  await expect(loadUserConfig('bogus', {})).rejects.toThrow(/You need an `apiKey`/);
+  await expect(loadUserConfig('bogus', {})).rejects.toThrow(/Missing `apiKey` and `apiSecret` in your Happo config/);
+});
+
+it('yells if apiKey is missing', async () => {
+  requireRelative.mockImplementation(() => ({
+    apiSecret: '2',
+    targets: {},
+  }));
+  await expect(loadUserConfig('bogus', {})).rejects.toThrow(/Missing `apiKey` in your Happo config/);
+});
+
+it('yells if apiSecret is missing', async () => {
+  requireRelative.mockImplementation(() => ({
+    apiKey: '1',
+    targets: {},
+  }));
+  await expect(loadUserConfig('bogus', {})).rejects.toThrow(/Missing `apiSecret` in your Happo config/);
 });
 
 it('yells if targets are missing', async () => {
@@ -101,7 +117,7 @@ describe('when CHANGE_URL is defined', () => {
   it('adds a log', async () => {
     await loadUserConfig('bogus', { CHANGE_URL: 'foo.bar' });
     expect(info.mock.calls[0][0]).toEqual(
-      'No `apiKey` or `apiSecret` found in config. Falling back to pull-request authentication.',
+      'Missing `apiKey` and `apiSecret` in Happo config. Falling back to pull-request authentication.',
     );
   });
 
@@ -146,7 +162,7 @@ describe('when GITHUB_EVENT_PATH is defined', () => {
         loadUserConfig('bogus', {
           GITHUB_EVENT_PATH: path.resolve(__dirname, 'github_push_event.json'),
         }),
-      ).rejects.toThrow(/You need an.*apiSecret/);
+      ).rejects.toThrow(/Missing `apiKey` and `apiSecret`/);
     });
   });
 });