bluetooth: Better disconnected error messages for GATT operations.

third_party/WebKit/LayoutTests/bluetooth/script-tests/descriptor/io-op-disconnect-called-during-error.js

 'use strict';
 promise_test(
     () => {
       let val = new Uint8Array([1]);
       return setBluetoothFakeAdapter('FailingGATTOperationsAdapter')
           .then(
               () => requestDeviceWithKeyDown(
                   {filters: [{services: [errorUUID(0xA0)]}]}))
           .then(device => device.gatt.connect())
           .then(gattServer => {
             return gattServer.getPrimaryService(errorUUID(0xA0))
                 .then(service => service.getCharacteristic(errorUUID(0xA1)))
                 .then(
                     characteristic =>
                         characteristic.getDescriptor(user_description.name))
                 .then(descriptor => {
                   let promise = assert_promise_rejects_with_message(
                       descriptor.CALLS([readValue()|writeValue(val)]),
                       new DOMException(
-                          'GATT Server disconnected while performing a GATT operation.',
+                        'GATT Server is disconnected. Cannot perform GATT operations. ' +
+                          '(Re)connect first with `device.gatt.connect`.',
                           'NetworkError'));
                   gattServer.disconnect();
                   return promise;
                 });
           });
     },
     'disconnect() called during a FUNCTION_NAME call that fails. ' +
         'Reject with NetworkError.');