UnboundLocalError when invalid SourceCode is provided

[benieric]

Describe the bug
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This error happens in the _prepare_train_script() method if code execution reaches this line without setting the execute_driver variable.

From some testing looks like this happens when an invalid SourceCode is provided to model trainer it results in an error like

UnboundLocalError: cannot access local variable 'execute_driver' where it is not associated with a value

For example:

source_code = SourceCode(entry_point="train.py")

model_trainer = ModelTrainer(source_code=source_code,...)
model_trainer.train() # would fail here

2 issues:

1. This method should have a final else to raise error with more informative message instead of falling through and failing later
2. The SourceCode class should fail sooner if user incorrectly tries to set entry_point instead of entry_script

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Expected behavior
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System information
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