we seem to be returning the wrong error code when adding an undefined alias to canonical_alias

[richvdh]

we're returning a 404 with {errcode: "M_NOT_FOUND", error: "Room alias #megolm:sw1v.org not found"}; matrix-org/matrix-spec-proposals#2432 (comment) suggests a different conclusion.

The upshot is that the error message in riot is unhelpful:

[richvdh]

related to #6898

[clokep]

@richvdh I believe this is working as expected. The validation code throw three errors:

1. M_INVALID_PARAM if the form of the alias is wrong.
2. M_NOT_FOUND if the alias is completely unknown.
3. M_BAD_ALIAS if the alias is found, but does not point to this room.

I think this matches the comment you pointed to, when taking into account the context of the conversation above it.

Note that the logic for this was implemented in #6971, although it is a bit hard to see:

1. M_INVALID_PARAM is raised from RoomAlias.from_string()
2. M_NOT_FOUND is raised via directory_handler.get_association()
3. M_BAD_ALIAS is raised directly.

[clokep]

This was fixed in #7109.