Remove default --file value and adjust error messages

[aheil-lanl]

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Description

This pull request stops --file from defaulting to inventory.yaml across all subcommands, and adds appropriate errors when --file is not defined.

I am targeting a confusing error message in the firmware subcommand, where if neither --file nor --hosts is defined the tool attempts to read inventory.yaml and fails with a confusing message if that file does not exist.

Type of Change

• Bug fix
• New feature
• Breaking change -- would break any workflows that expect file to default.
• Documentation update

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