[sidecar] better error handling for anthropic

[maan2003]

we were eating some errors

sample output

stdout:

anthropic::err({"type":"error","error":{"type":"rate_limit_error","message":"This request would exceed your organization’s rate limit of 200,000 input tokens per minute. For details, refer to: https://docs.anthropic.com/en/api/rate-limits; see the response headers for current usage. Please reduce the prompt length or the maximum tokens requested, or try again later. You may also contact sales at https://www.anthropic.com/contact-sales to discuss your options for a rate limit increase."}})

error message:

Error processing input: Reqwest error: HTTP status client error (429 Too Many Requests) for url (https://api.anthropic.com/v1/messages)