Removed erroneous PastEvent::id().
Split PastTest into PastKernelTest and PastWebTest.
Replaced _drupal_decode_exception().

Also _drupal_render_exception_safe(), and added \ namespace prefix to
some Exception references.
Moved save() from PastEvent to doSave() in controller.

The code makes sure arguments are saved.
Moved WebTest-dependent part of testSave() from kernel test to web test.