Improve error handling in 'gr-change-list-view'

- hide link 'Next' when an error occurs
- hide links 'Next' / 'Prev' initially
  (and thus property '_changes' is undefined)
- remove unused code

Change-Id: Ib6f77232481988b81aad8b996bfc9c9432c4d826

polygerrit-ui/app/elements/change-list/gr-change-list-view/gr-change-list-view.html

@@ -49,17 +49,17 @@ limitations under the License.
         }
       }
     </style>
-    <div class="loading" hidden$="[[!_loading]]">Loading...</div>
+    <div class="loading" hidden$="[[!_loading]]" hidden>Loading...</div>
-    <div hidden$="[[_loading]]">
+    <div hidden$="[[_loading]]" hidden>
       <gr-change-list
           changes="{{_changes}}"
           selected-index="{{viewState.selectedChangeIndex}}"
           show-star="[[loggedIn]]"></gr-change-list>
       <nav>
         <a href$="[[_computeNavLink(_query, _offset, -1, _changesPerPage)]]"
-           hidden$="[[_hidePrevArrow(_offset)]]">&larr; Prev</a>
+           hidden$="[[_hidePrevArrow(_offset)]]" hidden>&larr; Prev</a>
         <a href$="[[_computeNavLink(_query, _offset, 1, _changesPerPage)]]"
-           hidden$="[[_hideNextArrow(_changes.length, _changesPerPage)]]">
+           hidden$="[[_hideNextArrow(_loading, _changesPerPage)]]" hidden>
           Next &rarr;</a>
       </nav>
     </div>

polygerrit-ui/app/elements/change-list/gr-change-list-view/gr-change-list-view.js

@@ -123,20 +123,12 @@
       return href;
     },
 
-    _computeErrorHidden: function(loading, lastError) {
-      return loading || lastError == null;
-    },
-
-    _computeListHidden: function(loading, lastError) {
-      return loading || lastError != null;
-    },
-
     _hidePrevArrow: function(offset) {
-      return offset == 0;
+      return offset === 0;
     },
 
-    _hideNextArrow: function(changesLen, changesPerPage) {
+    _hideNextArrow: function(loading, changesPerPage) {
-      return changesLen < changesPerPage;
+      return loading || !this._changes || this._changes.length < changesPerPage;
     },
   });
 })();