make lr finder error message more helpful.

[leej3]

Fixes #3152

Description:
Small change to an error message to help users encountering type errors when using the fast ai learning rate finder. As mentioned in original issue, there's no compelling reason to update the docs. Helping users to transform the trainer output if that is the cause of the error should be sufficient.

[sweep-ai]

Apply Sweep Rules to your PR?

• Apply: All new business logic should have corresponding unit tests.
• Apply: Refactor large functions to be more modular.
• Apply: Add docstrings to all functions and file headers.

[vfdev-5]

To repro formatting issue:

  bash ./tests/run_code_style.sh install
  bash ./tests/run_code_style.sh lint

and to fix it :

  bash ./tests/run_code_style.sh fmt

[vfdev-5]

Thanks John, LGTM!