Raise errors if incompatible args are used with an executor

[jackgerrits]

Why are these changes needed?

If a user tries to use the old/existing way of configuring code execution with an executor it will not work. This raises an error to ensure the user knows it wont work.

Related issue number

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://microsoft.github.io/autogen/docs/Contribute#documentation to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 6 lines in your changes are missing coverage. Please review.

Project coverage is 46.79%. Comparing base (799a24e) to head (58311aa).

Files	Patch %	Lines
autogen/agentchat/conversable_agent.py	0.00%	3 Missing and 3 partials ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #1855       +/-   ##
===========================================
+ Coverage   36.03%   46.79%   +10.76%     
===========================================
  Files          63       63               
  Lines        6608     6614        +6     
  Branches     1456     1584      +128     
===========================================
+ Hits         2381     3095      +714     
+ Misses       4036     3269      -767     
- Partials      191      250       +59     

Flag	Coverage Δ
unittests	46.71% <0.00%> (+10.68%)	⬆️

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