[python-experimental] Minor doc update, code comments and exception handling

Created by: sebastien-rosset

This PR is not ready, I am opening to start discussion. I'm not sure yet what needs to be fixed.

An OAS document may have a schema that does not specify the "type" attribute.

Pet:
  type: object
  properties:
    name:
      type: string
    annotations:
      type: Annotations

Annotations:
  description: an arbitrary element that can represent any JSON schema,
     including the "null" type, an object, or an array.
     Notice the "type" attribute has not been set.
  nullable: true

In that case, the generated Python openapi_types() function will have entries such as:

'name': (str,),  # noqa: E501
# without nullable: true
'annotations': (object,),  # noqa: E501
# When nullable: true is specified:
'annotations': (object, none_type,),  # noqa: E501     

Without this PR change, in model_utils.py, COERCION_INDEX_BY_TYPE, when the order_response_types function is invoked, it will call COERCION_INDEX_BY_TYPE[class_or_instance] where class_or_instance is 'object', and because 'object' is not in COERCION_INDEX_BY_TYPE, an exception is raised. Also, because there was not check to determine if the key exists in the COERCION_INDEX_BY_TYPE dict, the raised exception was not user-friendly.

PR checklist

• Read the contribution guidelines.
• If contributing template-only or documentation-only changes which will change sample output, build the project before.
• Run the shell script(s) under ./bin/ (or Windows batch scripts under.\bin\windows) to update Petstore samples related to your fix. This is important, as CI jobs will verify all generator outputs of your HEAD commit, and these must match the expectations made by your contribution. You only need to run ./bin/{LANG}-petstore.sh, ./bin/openapi3/{LANG}-petstore.sh if updating the code or mustache templates for a language ({LANG}) (e.g. php, ruby, python, etc).
• File the PR against the correct branch: master, 4.3.x, 5.0.x. Default: master.
• Copy the technical committee to review the pull request if your PR is targeting a particular programming language.