fix parsing error of api error response (#381)

[vvatanabe]

Fix #381

Context:
Basically, it is a string. However, if the json schema of the parameters field is invalid, as shown below, it is likely to be an array.

$ curl https://api.openai.com/v1/chat/completions -u :$OPENAI_API_KEY -H 'Content-Type: application/json' -d '{
  "model": "gpt-3.5-turbo-0613",
  "messages": [
    {"role": "user", "content": "What is the weather like in Boston?"}
  ],
  "functions": [
    {
      "name": "get_current_weather",
      "parameters": {
        "type": "object"
      }
    }
  ]
}'

{
  "error":{
     "message":[
        "Invalid schema for function 'get_current_weather': In context=(), object schema missing properties"
     ],
     "type":"invalid_request_error",
     "param":null,
     "code":null
  }
}

[codecov]

Codecov Report

Merging #384 (e904f30) into master (f0770cf) will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #384      +/-   ##
==========================================
+ Coverage   95.22%   95.26%   +0.04%     
==========================================
  Files          17       17              
  Lines         670      676       +6     
==========================================
+ Hits          638      644       +6     
  Misses         22       22              
  Partials       10       10              

Impacted Files	Coverage Δ
error.go	100.00% <100.00%> (ø)

[vvatanabe]

@sashabaranov Please check it.

[vvatanabe]

This test has been moved to the newly added table test.

[sashabaranov]

Thank you!!