Add None checks to aws.get_s3_file

[garlic-os]

Description

Adds a check to ensure the value returned from aws.get_s3_file is not None in every place that it is used. If the value is None, execution is halted with an error message that better explains what went wrong.

Motivation and Context

Fixes #648.

How Has This Been Tested?

Added regression tests to ensure the proper messages are shown when a GUNW file is not produced. (TO DO)

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• I have added an explanation of what your changes do and why you'd like us to include them.
• I have written new tests for your core changes, as applicable.
• I have successfully ran tests with your changes locally.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.

[jlmaurer]

LGTM