Improve Ipyannotator API message error

[itepifanio]

Ipyannotator has an API to use its previously defined annotators. The API uses a pair of input/output and when this pair it's not configured the API should throw a friendly exception for the user.

Right now when a pair it's not correctly configured the API prints a friendly message (Pair (Annotator Input type: CustomInput, Annotator Output type: NoOutput) is not supported!) but also throws a random exception AttributeError: 'NoneType' object has no attribute 'get_annotator' this behavior can be reproduced using the following code:

from ipyannotator.mltypes import Input, Output
from ipyannotator.annotator import Annotator

class CustomInput(Input):
    pass

custom_input = CustomInput()
annotator = Annotator(custom_input)
annotator.explore()

The expected behavior it's:

• Ipyannotator throws a friendly custom exception (ex. PairUnsupported)
• Don't throw the AttributeError