Fix unbound local crash on handling aws lambda exception

[ghost]

Fix for #974

[ghost]

Don't know how to get the aws lambda tests to actually run. Until then, the coverage test will not pass.

[ghost]

Fix for exception:

    UnboundLocalError: local variable 'timeout_thread' referenced before assignment
    Traceback (most recent call last):
       File "/usr/local/lib/python3.8/dist-packages/sentry_sdk/integrations/aws_lambda.py", line 160, in sentry_handler
        if timeout_thread:

[untitaker]

Thanks!