Raise 400 error for invalid contributor param type

[kellyi]

Overview

Raise a 400 error if not all of the contributor querystring parameters
are integers, sending back a detailed error response about the type.

The other custom parameters already expect strings, so they don't fail
in the same way, and the pagination params have their own error
handling. Therefore we don't need to add similar handling to the other
errors.

Connects #424

Demo

Testing Instructions

• resetdb then processfixtures
• serve this branch
• visit http://localhost:8081/api/docs/#!/facilities/facilities_list and try entering a string value in the contributors input, then search and verify that it returns 400
• change the string value to and int, like 2, and verify that it returns 200 and that you see a FeatureCollection
• visit the main app on 6543 then search for facilities in various ways and verify that that also still works

Checklist

• fixup! commits have been squashed
• CI passes after rebase
• CHANGELOG.md updated with summary of features or fixes, following Keep a Changelog guidelines

[rajadain]

+1 tested, works well.

I tried briefly to see if this could be done with validators, but they would require having a serializer for the payload which we currently don't.

On one hand that feels like a heavier lift. On another, maybe it would be a good idea to have well-defined parameters. Maybe for a future task.

[kellyi]

Good point. It appears that we can do this using a serializer on its own -- https://tinyendian.com/articles/parsing-query-parameters-in-rest-framework/ -- so I will update this to do that.

[kellyi]

bbf02ed is an amended commit that users a custom serializer + the built-in ValidationError for handling errors related to the queryparams

[kellyi]

Rebased on develop

[rajadain]

+1 clean, elegant solution.

[kellyi]

Thanks!

[jwalgran]

I was delighted by how simple and declarative the serializer solution was but I wondered how it was dealing with the fact that a query string argument could appear multiple times. This lead me to an edge case:

This returns 400 like we want
http://localhost:8081/api/facilities/?contributors=1&contributors=two

This, unfortunately, still returns 500
http://localhost:8081/api/facilities/?contributors=one&contributors=2

[kellyi]

This lead me to an edge case:

Oof, I merged the PR before I saw this. Will make a new PR to fix.