Bug 1067550: Add support for new get-involved page.

Not added to docs because custom endpoint for only one site.

@Osmose r?
but the same error codes happen for several errors, and the `desc` is the only differentiator that tells me that the correct error has fired and that it's not just that my test is bad and forgot another thing. I could add a bunch of error codes, but I don't think the client particularly cares.