[MAINTENANCE] Raise exceptions on init in cloud mode

[donaldheppner]

Changes proposed in this pull request:

• DataContext raises an exception on initialization when in Cloud Mode

Definition of Done

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• [ x ] My code follows the Great Expectations style guide
• [ x ] I have performed a self-review of my own code
• [ x ] I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• [ x ] I have added unit tests where applicable and made sure that new and existing tests are passing.
• [ x ] I have run any local integration tests and made sure that nothing is broken.

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