Return array of devfile errors rather than a single string

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What does this MR do and why?

Fixes #564551

References

Screenshots or screen recordings

Before	After
Rails Console	Rails console

How to set up and validate locally

1. Start GDK and go to GraphQL explorer at: gdk.test:3000/-/graphql-explorer
2. Send the mutation (In comments) to GraphQL explorer and check the returned errors value
3. (Alternative): Use bin/rails c and use the following setup values (In comments) get the errors value

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #564551