fix: surface provider API errors instead of showing empty response

[superShen0916]

Summary

Fixes #3264 — When OpenRouter or other providers return rate-limit (429) or insufficient-credits (402) errors, LiteLLM may wrap the error in a response object rather than raising an exception. Previously, aider showed "0 received" with no explanation, leaving users confused about why no response was generated.

Changes

1. Detect API errors in completion responses (send() in base_coder.py): After send_completion returns, check if the response has an error field (populated by LiteLLM for API-level errors). If present, display the error message via tool_error and return early — avoiding the misleading token report.

2. Improve empty-response warning (show_send_output_stream): Replace the generic "Empty response received from LLM. Check your provider account?" with a more actionable message that mentions rate limits, insufficient credits, and provider availability as likely causes, and suggests --verbose for debugging.

Test plan

• The error path only activates when completion.error is populated (currently for provider errors that LiteLLM wraps in a response)
• Successful responses are unaffected — the error check returns None/falsy for normal completions
• The improved warning message is a string change only

Risk

Low. The error check is defensive and only fires when the provider returns an explicit error object. All existing success paths are unchanged.

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