fix: DBTP-1373 - Handle SchemaErrors from platform config validation gracefully

[ksugden]

Addresses DBTP-1373

When a config validation failed during platform-helper commands, an exception was thrown, resulting in a large traceback and an obscured reason for the schema validation error.

This PR catches the schema validation error and exits platform-helper gracefully, outputting only the schema validation error message.

The tests are updated for the expected SystemExit exception.

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Checklist:

Title:

• Scope included as per conventional commits
• Ticket reference included (unless it's a quick out of ticket thing)

Description:

• Link to ticket included (unless it's a quick out of ticket thing)
• Includes tests (or an explanation for why it doesn't)
• If the work includes user interface changes, before and after screenshots included in description
• Includes any applicable changes to the documentation in this code base
• Includes link(s) to any applicable changes to the documentation in the DBT Platform Documentation (can be to a pull request)

Tasks:

• Trigger the pull request regression tests for this branch and confirm that they are passing