Fix client data checker return

[tsmith023]

This PR fixes a bug raised in https://github.com/weaviate/typescript-client/issues/63 where a >400 error code returned from a HEAD request does not propagate correctly to the user. Also contained are small type improvements to help with DevEx and increased ease of understanding of the codebase.

@parkerduckworth :grin:

[weaviate-git-bot]

Great to see you again! Thanks for the contribution.

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[parkerduckworth]

Thanks @tsmith023 !