Maintenance: Unify error constructors between `homeserver` and `api` packages

[turt2live]

They both have similar-but-different error handling code. We should extract it and make it available to both.

https://github.com/matrix-org/policyserv/blob/4d57eb3073d4b71978fa11995e42a3218799b723/api/errors.go

https://github.com/matrix-org/policyserv/blob/4d57eb3073d4b71978fa11995e42a3218799b723/homeserver/errors.go