docs: document error handling in the README

[erimicel]

Adds an "Error handling" section to the README. This is the README part of #60, split out on its own.

It also corrects the example from #60. Errors are built with new(response: response), so e.data is a Hash and the Faraday response is at e.data[:response]. The #60 example called e.data.status, which raises NoMethodError.

Changes

• How to read the message, HTTP status and body from a raised Typesense::Error.
• Which exception class each status code maps to.
• Network failures that exhaust retries are re-raised as the original Faraday/Errno exception, not as a Typesense::Error.

Verification

Ran the README example against master with a stubbed 404 response: e.message is "Not Found", e.data[:response].status is 404, and e.data[:response].body is the raw JSON body.

PR Checklist

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