Add cancel to device management network failure alert dialog

Closes #4844

src/org/thoughtcrime/securesms/DeviceListFragment.java

@@ -146,6 +146,20 @@ public class DeviceListFragment extends ListFragment
         getLoaderManager().initLoader(0, null, DeviceListFragment.this);
       }
     });
+
+    builder.setNegativeButton(android.R.string.cancel, new DialogInterface.OnClickListener() {
+      @Override
+      public void onClick(DialogInterface dialog, int which) {
+        DeviceListFragment.this.getActivity().onBackPressed();
+      }
+    });
+    builder.setOnCancelListener(new DialogInterface.OnCancelListener() {
+      @Override
+      public void onCancel(DialogInterface dialog) {
+        DeviceListFragment.this.getActivity().onBackPressed();
+      }
+    });
+
     builder.show();
   }