Show helpful message when adding group member 404s

Previosuly, 404 errors showed a generic error message, making it unclear
what could have gone wrong. For security reasons, we don't want people
to know if the group does not exist or if the group exists but the user
does not have permission. This change updates the error message to
reflect both of these possibilities.

Bug: Issue 8252
Change-Id: I6940d53b342a81ca10ec78977de4ec548fd10a6f

polygerrit-ui/app/elements/admin/gr-group-members/gr-group-members.js

@@ -15,6 +15,8 @@
   'use strict';
 
   const SUGGESTIONS_LIMIT = 15;
+  const SAVING_ERROR_TEXT = 'Group may not exist, or you may not have '+
+      'permission to add it';
 
   const URL_REGEX = '^(?:[a-z]+:)?//';
 
@@ -186,7 +188,16 @@
 
     _handleSavingIncludedGroups() {
       return this.$.restAPI.saveIncludedGroup(this._groupName,
-          this._includedGroupSearch)
+          this._includedGroupSearch, err => {
+            if (err.status === 404) {
+              this.dispatchEvent(new CustomEvent('show-alert', {
+                detail: {message: SAVING_ERROR_TEXT},
+                bubbles: true,
+              }));
+              return err;
+            }
+            throw Error(err.statusText);
+          })
           .then(config => {
             if (!config) {
               return;

polygerrit-ui/app/elements/admin/gr-group-members/gr-group-members_test.html

@@ -183,6 +183,24 @@ limitations under the License.
       });
     });
 
+    test('add included group 404 shows helpful error text', () => {
+      element._groupOwner = true;
+
+      const memberName = 'bad-name';
+      const alertStub = sandbox.stub();
+      element.addEventListener('show-alert', alertStub);
+
+      sandbox.stub(element.$.restAPI, 'saveGroupMembers',
+          () => Promise.reject({status: 404}));
+
+      element.$.groupMemberSearchInput.text = memberName;
+      element.$.groupMemberSearchInput.value = 1234;
+
+      return element._handleSavingIncludedGroups().then(() => {
+        assert.isTrue(alertStub.called);
+      });
+    });
+
     test('_getAccountSuggestions empty', () => {
       return element._getAccountSuggestions('nonexistent').then(accounts => {
         assert.equal(accounts.length, 0);

polygerrit-ui/app/elements/shared/gr-rest-api-interface/gr-rest-api-interface.js

@@ -437,12 +437,16 @@
           .then(response => this.getResponseObject(response));
     },
 
-    saveIncludedGroup(groupName, includedGroup) {
+    saveIncludedGroup(groupName, includedGroup, opt_errFn) {
       const encodeName = encodeURIComponent(groupName);
       const encodeIncludedGroup = encodeURIComponent(includedGroup);
       return this.send('PUT',
-          `/groups/${encodeName}/groups/${encodeIncludedGroup}`)
-          .then(response => this.getResponseObject(response));
+          `/groups/${encodeName}/groups/${encodeIncludedGroup}`, null,
+          opt_errFn).then(response => {
+            if (response.ok) {
+              return this.getResponseObject(response);
+            }
+          });
     },
 
     deleteGroupMembers(groupName, groupMembers) {