Refactor Session.setup to handle promise errors

It's possible for an `UnhandledPromiseError` to raise when restoring the session. This doesn't raise to the caller of the library but does occur as a caught exception in the browser, which in turn can be picked up by error tracking code such as sentry.

This fixes #2358 which has more specific detail around the issue and how to reproduce it.

packages/ember-simple-auth/addon/services/session.js

@@ -391,13 +391,12 @@ export default Service.extend(Evented, {
  @method setup
  @public
  */
- async setup() {
+ setup() {
  this._setupIsCalled = true;
- try {
- this._setupHandlers();
- await this.session.restore();
- } catch (error) {
+ this._setupHandlers();
+
+ return this.session.restore().catch(() => {
  // If it raises an error then it means that restore didn't find any restorable state.
- }
+ });
  },
 });