REF: DX-1182 Prevent TypeError when API returns error responses (#47)

* Prevent TypeError when API returns error responses

Improvements: DX-1182

Previously, when the API returned non-404 error responses (401, 500, etc.),
the code would attempt to call .map() on the error object, resulting in:
"TypeError: responseBody.map is not a function"

Changes:
- Add validation for HTTP status codes before processing response
- Check if responseBody is an array before calling .map()
- Extract and include error details from API error responses
- Provide clear, actionable error messages

Example error messages:
- "Failed to fetch revisions (HTTP 401): unauthorized"
- "Expected array of revisions, but got object: {...}"

This fix addresses transient API failures and authentication issues that
previously resulted in cryptic TypeErrors.

* Address issues pointed out by automated review.

Author: scraswell

lib/rest-client.js

@@ -109,13 +109,40 @@ module.exports = CoreObject.extend({
     }
 
     return denodeify(this._request.get)({ uri, qs: qs }).then(function (response) {
-      var responseBody =
-        useReleaseEndpoint && isSummaryEndpoint ? response.body.filter((rev) => rev.app === appName) : response.body;
-
+      // Handle 404 as empty revision list
       if (response.statusCode == 404) {
         return [];
       }
 
+      // Handle non-2xx status codes
+      if (response.statusCode < 200 || response.statusCode >= 300) {
+        var errorMessage = 'Failed to fetch revisions (HTTP ' + response.statusCode + ')';
+
+        // Try to extract error details from response body
+        if (response.body && response.body.errors && Array.isArray(response.body.errors)) {
+          var errorDetails = response.body.errors
+            .map(function (err) {
+              return err.detail || err.message || JSON.stringify(err);
+            })
+            .join(', ');
+          errorMessage += ': ' + errorDetails;
+        } else if (response.body && typeof response.body === 'object') {
+          errorMessage += ': ' + JSON.stringify(response.body);
+        }
+
+        return Promise.reject(new Error(errorMessage));
+      }
+
+      if (!Array.isArray(response.body)) {
+        var bodyType = response.body === null ? 'null' : typeof response.body;
+        return Promise.reject(
+          new Error('Expected array of revisions, but got ' + bodyType + ': ' + JSON.stringify(response.body)),
+        );
+      }
+
+      var responseBody =
+        useReleaseEndpoint && isSummaryEndpoint ? response.body.filter((rev) => rev.app === appName) : response.body;
+
       return responseBody.map(function (revision) {
         // ember-cli-deploy-display-revisions expects the timestamp to be either
         // a JS Date object or the number of milliseconds since 1970-01-01T00:00:00