Replace mocks in test_CatalogManager_load_invalid_model

[charles-turner-1]

Is your feature request related to a problem? Please describe.

The tests adde for more thorough catalog manager error handling in #255 relied on mocking exceptions raised from intake-dataframe-catalog.

I'm not really a fan of doing this as we're making assumptions about the error handling in that package, rather than the data, and so I would like to replace these mocks with a proper test of system response to data that should lead to the mocked exceptions being raised from intake-dataframe-catalog.

This will take a bit of time to do, so I'm leaving it as an issue for now & will replace the mocks after release 1.0.0