🥅 Handle error details on inactive rpc errors

[evaline-ju]

Previously #372 was added to give connection errors a more understandable user message, but this obscured some unary generation validation errors

In this PR:

• Serializing err.details was leading to JSON errors - this should be err.details()
• Non-connection errors such as validation errors [invalid argument] were getting surfaced with the inactive RPC connection message incorrectly

Test command with this fix

grpcurl -plaintext -H "mm-model-id: [model-id]" -d '{
  "stopSequences": [],
  "text": "sad",
  "tokenLogprobs": true
}' localhost:8085 caikit.runtime.Nlp.NlpService.TextGenerationTaskPredict

[gkumbhat]

🤦 thanks for fixing it. LGTM.