Have the fetch tag dialog show a warning for robot accounts without access

Before this change, we'd show the squash pulling command with the proper credentials, but it then 403s on the end user.

static/directives/fetch-tag-dialog.html

@@ -56,6 +56,13 @@
           </table>
 
           <div class="cor-loader-inline" ng-if="currentEntity && !currentRobot"></div>
+
+          <div class="co-alert co-alert-warning"
+               ng-if="currentRobotHasPermission === false">
+            Warning: Robot account <strong>{{ currentRobot.name }}</strong> does not have
+            read permission on this repository, so the command below will fail with an authorization error.
+          </div>
+
           <div ng-if="getCommand(currentFormat, currentRobot)">
             Command:
             <pre class="command">{{ getCommand(currentFormat, currentRobot) }}</pre>