session: execute browser teardown only once (#1390)

The browser teardown in `airgun.session.Session.__exit__` should be executed only once.

Because when `Session` is nested in multiple context managers(*),
the `__exit__` method is called for each one
and this causes that the second and further `webdriver.quit()` call will fail,
because the browser session was already stopped.

(*) for example in the robottelo `session` fixture in `robottelo/pytest_fixtures/core/ui.py`.

airgun/session.py

@@ -232,7 +232,7 @@ def __exit__(self, exc_type, exc_value, traceback):
             not risen not to shadow real session result.
         """
         if self.browser is None:
-            # browser was never started, don't do anything
+            # browser hasn't been started or was already closed, don't do anything
             return
         LOGGER.info('Stopping UI session %r for user %r', self.name, self._user)
         passed = True if exc_type is None else False
@@ -242,7 +242,7 @@ def __exit__(self, exc_type, exc_value, traceback):
         except Exception as err:  # - TODO: fix bare except
             LOGGER.exception(err)
         finally:
-            self._factory.finalize(passed)
+            self.browser = self._factory.finalize(passed)
 
     def _open(self, entity):
         """Initializes requested entity. If this is first time session