Add meaningful error message if response queues are not initialized

[rasbt]

Before submitting

• Was this discussed/agreed via a Github issue? (no need for typos and docs improvements)
• Did you read the contributor guideline, Pull Request section?
• Did you make sure to update the docs?
• Did you write any new necessary tests?

What does this PR do?

Adds a meaningful error message if the response queue has not been initialized. Otherwise, you see an

AttributeError: 'LitServer' object has no attribute 'response_queue

PR review

Anyone in the community is free to review the PR once the tests have passed.
If we didn't discuss your PR in GitHub issues there's a high chance it will not be merged.

Did you have fun?

Make sure you had fun coding 🙃

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 79%. Comparing base (75d04ca) to head (c4dfbd8).

Additional details and impacted files

@@         Coverage Diff         @@
##           main   #191   +/-   ##
===================================
- Coverage    79%    79%   -0%     
===================================
  Files        13     13           
  Lines      1036   1038    +2     
===================================
- Hits        821    819    -2     
- Misses      215    219    +4     

[aniketmaurya]

hi @rasbt, could you also provide a script or example where you encountered this issue? Ideally we don't want this issue to come in the first place.