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.

Author: scraswell

lib/rest-client.js

@@ -109,11 +109,37 @@ module.exports = CoreObject.extend({
     }
 
     return denodeify(this._request.get)({ uri, qs: qs }).then(function (response) {
+      // 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));
+      }
+
       var responseBody =
         useReleaseEndpoint && isSummaryEndpoint ? response.body.filter((rev) => rev.app === appName) : response.body;
 
-      if (response.statusCode == 404) {
-        return [];
+      // Validate that responseBody is an array
+      if (!Array.isArray(responseBody)) {
+        var bodyType = responseBody === null ? 'null' : typeof responseBody;
+        return Promise.reject(
+          new Error('Expected array of revisions, but got ' + bodyType + ': ' + JSON.stringify(responseBody))
+        );
       }
 
       return responseBody.map(function (revision) {