if error response does not contain message set error to Unknown

fixes issues addressed here #17
with additional tests

lib/api-request-test.js

@@ -47,6 +47,27 @@ describe('api', () => {
     });
   });
 
+  describe('when the response status is not 200', () => {
+    describe('and the response JSON is not formatted correctly', () => {
+      it('does not blow up', (done) => {
+        const apiRequest = api.apiRequest(HOST, API_KEY, USER_AGENT);
+        const METHOD = 'get',
+          ENDPOINT = '/whoopie-do',
+          OPTIONS = { endpoint: ENDPOINT },
+          BAD_JSON_RESPONSE_BODY = { i: 'sorry' };
+
+        nock(HOST)[METHOD](ENDPOINT)
+        .reply(400, BAD_JSON_RESPONSE_BODY);
+
+        apiRequest(METHOD, OPTIONS, (err, body) => {
+          expect(body).toBeUndefined()
+          expect(err.message).toBe('Unknown error')
+          done();
+        });
+      });
+    });
+  });
+
   describe('when the response is not JSON formatted', () => {
     describe('and the response status is 200', () => {
       it('does not blow up', (done) => {

lib/api-request.js

@@ -19,7 +19,11 @@ module.exports = {
       } else {
         try {
           const parsedBody = JSON.parse(body);
-          cb(new Error(parsedBody.error.message));
+          if (!parsedBody || !parsedBody.error || !!parsedBody.error.message) {
+            cb(new Error('Unknown error'));
+          } else {
+            cb(new Error(parsedBody.error.message));
+          }
         } catch (e) {
           cb(new Error('The response from the server was not a JSON'));
         }