get_variable_model_to_mip_mappings should raise a single exception if a mapping cannot be found

Migrated Jira Issue CDDSO-29
Jira creation date 21/04/2022 15:23
-------
There are currently two ways that retrieving a //model to MIP mapping// using `get_variable_model_to_mip_mapping` can raise an exception; if there is no //model to MIP mapping// a `configparser.Error` exception is raised and if there is a //model to MIP mapping// but not for the required //MIP table// a `RuntimeError` exception is raised (see ticket:623#comment:12). To make it easier for the user of `get_variable_model_to_mip_mapping` to catch the exception, a single exception should instead be raised (and documented appropriately).
-------

Locate get_variable_model_to_mip_mapping and inspect the two exception paths described in this issue and ticket 623 comment 12. Confirm how each missing-mapping case is currently reported, then make the behavior and its documentation consistent around one exception type.

Written by the indexing model from the issue text.