Add appropriate response to Models().SKModel().load_model('<model_id>', connection_object) when non-existent model is specified.

[vc1492a]

Currently, calling Models().SKModel().load_model() results in a KeyError if the ID of a non-existent model is specified, see below:

WARNING:root:<Response [404]>
{'description': 'Error retrieving model from Elasticsearch.'}
---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
<ipython-input-10-8aa4cd950881> in <module>()
      1 for m in list_models():
----> 2     model = Models().SKModel().load_model('bbe4830f5eb24a73a299b720eef58ccd', s)
      3     print(model)
      4 
      5 

~/Files/Projects/5X/PRS Form Recommender/api/src/henosis/Henosis/model.py in load_model(self, model_id, server_config)
    344             print(model_info)
    345 
--> 346             self.id = model_info['models']['id']
    347             self.deployed = model_info['models']['deployed']
    348             self.call_count = model_info['models']['callCount']

KeyError: 'models'

The aim here is to improve the response to the error, e.g. something along the lines of 'Specified model ID not present in Henosis instance.' and by also electing to not attempt to load the model if an error occurs in the API (currently, an error presented in the API response does not stop Henosis from attempting to return a model object).