bluetooth: Better disconnected error messages for GATT operations.

third_party/WebKit/LayoutTests/bluetooth/script-tests/characteristic/descriptor-garbage-collection-ran-during-error.js

 'use strict';
 promise_test(() => {
   let promise;
   return setBluetoothFakeAdapter('DisconnectingHealthThermometerAdapter')
     .then(() => requestDeviceWithKeyDown({
       filters: [{services: ['health_thermometer']}]}))
     .then(device => device.gatt.connect())
     .then(gattServer => gattServer.getPrimaryService('health_thermometer'))
     .then(service => service.getCharacteristic('measurement_interval'))
     .then(characteristic => {
       promise = assert_promise_rejects_with_message(
         /* 0x0101 doesn't exist in this characteristic */
         characteristic.CALLS([
           getDescriptor(0x0101)|
           getDescriptors()|
           getDescriptors(0x0101)[UUID]]),
         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'));
       // Disconnect called to clear attributeInstanceMap and allow the
       // object to get garbage collected.
       characteristic.service.device.gatt.disconnect();
     })
     .then(runGarbageCollection)
     .then(() => promise);
 }, 'Garbage Collection ran during FUNCTION_NAME call that fails. ' +
    'Should not crash');