fix json marshaling error response of azure openai (#343)

[vvatanabe]

Fix #343

Azure OpenAI error response seems to be type field missing sometimes. Error (unexpected end of JSON input) when parsing type in APIError.UnmarshalJSON function.

Example error response:

# Azure OpenAI
{
   "error":{
      "code":"429",
      "message":"Requests to the Creates a completion for the chat message Operation under Azure OpenAI API version 2023-03-15-preview have exceeded token rate limit of your current OpenAI S0 pricing tier. Please retry after 17 seconds. Please go here: https://aka.ms/oai/quotaincrease if you would like to further increase the default rate limit."
   }
}

Note:
I also considered changing the type field to a pointer, but decided against it because that would be a breaking changes.

[codecov]

Codecov Report

Merging #345 (fe1edcd) into master (6830e00) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #345   +/-   ##
=======================================
  Coverage   93.65%   93.66%           
=======================================
  Files          17       17           
  Lines         646      647    +1     
=======================================
+ Hits          605      606    +1     
  Misses         30       30           
  Partials       11       11           

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

[vvatanabe]

@sashabaranov Please check it.

[sashabaranov]

Thank you! 🙌🏻