feat: add a `__str__` to ActionFailed, for better unexpected failure output

[tonyandrewmeyer]

Add a __str__ to ops.testing.ActionFailed so that an uncaught event.fail() in tests produces a more informative traceback.

Currently, event.fail() and event.fail(message) will look somewhat like:

Traceback (most recent call last):
  File "/tmp/1204.py", line 18, in <module>
    harness.run_action("act")
  File "/home/tameyer/code/operator/ops/testing.py", line 2001, in run_action
    raise ActionFailed(
ops.testing.ActionFailed

After this PR, event.fail() will look like:

Traceback (most recent call last):
  File "/tmp/1204.py", line 19, in <module>
    harness.run_action("act")
  File "/home/tameyer/code/operator/ops/testing.py", line 2006, in run_action
    raise ActionFailed(
ops.testing.ActionFailed: Event handler called `fail()` with no additional details.

and event.fail("Message") will look like:

Traceback (most recent call last):
  File "/tmp/1204.py", line 18, in <module>
    harness.run_action("act")
  File "/home/tameyer/code/operator/ops/testing.py", line 2006, in run_action
    raise ActionFailed(
ops.testing.ActionFailed: Message

Fixes #1204