HTTPoison error cases aren't handled in Stream.new/1

[Brad Folkens]

When handling a streaming request, OpenAI.Client#91 may return %HTTPoison.Error{reason: _, id: nil}, which then causes the following:

** (FunctionClauseError) no function clause matching in anonymous fn/1 in OpenAI.Stream.new/1

Having looked through the OpenAI.Stream module, and to preserve backward compatibility, I propose we handle the error case in the OpenAI.Stream.new/1 resource and return the error as a stream item, similar to the %{"status" => :error} pattern already present when non 200 status codes are received.