Raise all model warnings as issues in the data pipeline

[aflag]

Whenever we encounter an unexpected condition in the simulation (eg.: missing nodes in graph, missing connections, missing weights, etc), an issue should be logged in the data pipeline api.

Whenever we log an warning, we should instead create an issue such as

https://github.com/ScottishCovidResponse/data_pipeline_api/blob/e7059c4c0fbfc697de861a220d24bb0d8fece54b/data_pipeline_api/standard_api.py#L31

that should be attached back to the output:

https://github.com/ScottishCovidResponse/simple_network_sim/blob/master/simple_network_sim/sampleUseOfModel.py#L69

[github-actions[bot]]

Heads up @magicicada @bobturneruk @aflag @WPettersson @alex-konovalov @may1066 @mrow84 - the "Simple Network Sim" label was applied to this issue.