Ensure error conditions always roll back failed transactions

this was observed in augur.tasks.github.contributors.process_contributors

Stack Trace

```
Traceback (most recent call last):
File "/augur/.venv/lib/python3.11/site-packages/celery/app/trace.py", line 453, in trace_task
R = retval = fun(*args, **kwargs)
^^^^^^^^^^^^^^^^^^^^
File "/augur/.venv/lib/python3.11/site-packages/celery/app/trace.py", line 736, in __protected_call__
return self.run(*args, **kwargs)
^^^^^^^^^^^^^^^^^^^^^^^^^
File "/augur/augur/tasks/github/contributors.py", line 28, in process_contributors
key_auth = GithubRandomKeyAuth(logger)
^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/augur/augur/tasks/github/util/github_random_key_auth.py", line 17, in __init__
github_api_keys = GithubApiKeyHandler(logger).keys
^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/augur/augur/tasks/github/util/github_api_key_handler.py", line 38, in __init__
self.config_key = self.get_config_key()
^^^^^^^^^^^^^^^^^^^^^
File "/augur/augur/tasks/github/util/github_api_key_handler.py", line 60, in get_config_key
return get_value("Keys", "github_api_key")
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/augur/augur/application/db/lib.py", line 42, in get_value
config_setting = execute_session_query(query, 'one')
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/augur/augur/application/db/util.py", line 43, in execute_session_query
return catch_operational_error(func)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/augur/augur/application/db/util.py", line 21, in catch_operational_error
return func()
^^^^^^
File "/augur/.venv/lib/python3.11/site-packages/sqlalchemy/orm/query.py", line 2798, in one
return self._iter().one() # type: ignore
^^^^^^^^^^^^
File "/augur/.venv/lib/python3.11/site-packages/sqlalchemy/orm/query.py", line 2847, in _iter
result: Union[ScalarResult[_T], Result[_T]] = self.session.execute(
^^^^^^^^^^^^^^^^^^^^^
File "/augur/.venv/lib/python3.11/site-packages/sqlalchemy/orm/session.py", line 2306, in execute
return self._execute_internal(
^^^^^^^^^^^^^^^^^^^^^^^
File "/augur/.venv/lib/python3.11/site-packages/sqlalchemy/orm/session.py", line 2188, in _execute_internal
result: Result[Any] = compile_state_cls.orm_execute_statement(
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/augur/.venv/lib/python3.11/site-packages/sqlalchemy/orm/context.py", line 293, in orm_execute_statement
result = conn.execute(
^^^^^^^^^^^^^
File "/augur/.venv/lib/python3.11/site-packages/sqlalchemy/engine/base.py", line 1416, in execute
return meth(
^^^^^
File "/augur/.venv/lib/python3.11/site-packages/sqlalchemy/sql/elements.py", line 516, in _execute_on_connection
return connection._execute_clauseelement(
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/augur/.venv/lib/python3.11/site-packages/sqlalchemy/engine/base.py", line 1639, in _execute_clauseelement
ret = self._execute_context(
^^^^^^^^^^^^^^^^^^^^^^
File "/augur/.venv/lib/python3.11/site-packages/sqlalchemy/engine/base.py", line 1812, in _execute_context
conn = self._revalidate_connection()
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/augur/.venv/lib/python3.11/site-packages/sqlalchemy/engine/base.py", line 669, in _revalidate_connection
self._invalid_transaction()
File "/augur/.venv/lib/python3.11/site-packages/sqlalchemy/engine/base.py", line 659, in _invalid_transaction
raise exc.PendingRollbackError(
sqlalchemy.exc.PendingRollbackError: Can't reconnect until invalid transaction is rolled back. Please rollback() fully before proceeding (Background on this error at: https://sqlalche.me/e/20/8s2b)

```

we need to be more consistent about explicitly rolling things back on failure, so the un-rollled-back transaction doesnt just stay open in the db session for an error to happen when that session is released to the connection pool for some other part of the app to use