fix: handle null ErrorObject in raise_callable_error

[rarepolz]

Update raise_callable_error method to accept optional message parameter and raise CallableRuntimeError when error is None.

Issue #, if available:
closes #35
Description of changes:

• Updated raise_callable_error() method
  • Now accepts optional msg parameter and raises CallableRuntimeError instead of DurableExecutionsError when ErrorObject is None
• Added null-safe error handling
  • Method provides default "Unknown error" message or uses custom message when no ErrorObject exists
• Updated tests
  • Modified existing test and added new test case for custom message parameter functionality

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[ghost]

LGTM!