Refactored MXNet/PyTorch/TF Exceptions, and End-of-training log directory check

[jleeleee]

Description of changes:

Refactored all exception types under the smdebug/mxnet, smdebug/pytorch, and smdebug/tensorflow directories to return the SMDebugError exception type. Also changed the logic of the end of training log directory existence check in smdebug/core/hook_utils.py for the writer to return a boolean instead of raising an exception that gets caught later on, to make it clear that exceptions are generally meant to flow up to the customer, not to be used as a check.

Style and formatting:

I have run pre-commit install && pre-commit run --all-files to ensure that auto-formatting happens with every commit.

Issue number, if available

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