[trainer] make failure to find a resume checkpoint fatal + tests
As a follow up to https://github.com/huggingface/transformers/pull/10760 this PR:
- makes a failure to find a valid checkpoint to resume from fatal - when an explicit `resume_from_checkpoint` was passed 
- extends `test_can_resume_training` to validate this change and also the boolean `resume_from_checkpoint` case.
- adds a small `test_can_resume_training` refactoring - so it's easy to see they are the same args on each invocation.

@sgugger 