Raise NotConnectedError in UpdateHandler.check_connection

[hmpf]

… instead of AttributeError when something nasty has happened to the session object.

For first part of #68

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.

Project coverage is 70.25%. Comparing base (f13d7ad) to head (614f019).

Files	Patch %	Lines
src/zinolib/controllers/zino1.py	0.00%	3 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #71 +/- ## ========================================== - Coverage 70.35% 70.25% -0.10% ========================================== Files 13 13 Lines 1464 1466 +2 ========================================== Hits 1030 1030 - Misses 434 436 +2 ```

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[hmpf]

I am not sure it is a sufficient fix for first part of #68. It addresses the traceback specifically yes, but I see several more places in UpdateHandler code where same error might occur (The list is not exhaustive!):

1. `connect()` in UpdateHandler checks EventManager's session **before** calling the now fixed `check_connection`

Fair.

Instead of always returning the session object, connect_push_channel will raise a "NotConnectedError" if the connection could not be made.

2. `update()` and `remove()` in UpdateHandler call EventManager (including manager's methods that don't call `_verify_session`) yet never call `check_connection`

Not applicable.

update calls self.manager.get_updated_event_for_id(event_id) which eventually calls _verify_session before it tries talking on the net.

remove does not need a working connection, it only manipulates the local dict of events and the removed_ids set.

3. more?

Not in UpdateHandler.

Furthermore, while first part in #68 was exposed when calling UpdateHandler, it concerns the session from EventManager. Shouldn't we address the problem in EventManager?

SessionAdapter actually. See new commit.

[hmpf]

Last commit removed.