refactor: implement review suggestions

- Extract model conversion logic to _get_llm_instance helper method
- Improve error message clarity
- Simplify LLM instance creation in CrewEvaluator

Co-Authored-By: Joe Moura <joao@crewai.com>

src/crewai/utilities/evaluators/crew_evaluator_handler.py

@@ -36,10 +36,7 @@ class CrewEvaluator:
 
     def __init__(self, crew, llm: Union[str, LLM]):
         self.crew = crew
-        if isinstance(llm, str):
-            self.llm = LLM(model=llm)
-        else:
-            self.llm = llm
+        self.llm = LLM(model=llm) if isinstance(llm, str) else llm
         self._telemetry = Telemetry()
         self._setup_for_evaluating()