user-not-ready error from /dev/api/account is not specific enough

[Colin Watson]

_check_developer_preconditions responds with the same error code (user-not-ready) for the two cases of an unsigned developer agreement or an unset developer namespace. However, it's possible and desirable for a client to handle these and guide the user through any missing steps: build.snapcraft.io does this automatically, and I believe that the snapcraft team would like to. Unfortunately, at the moment that requires parsing the human-readable error message, which has led to some fairly predictable problems (see https://github.com/canonical-websites/build.snapcraft.io/pull/979).

It would be helpful to add fixed machine-readable codes for these errors so that we can make this more reliable. I wouldn't recommend changing the main error code from user-not-ready as client code is already relying on that, but perhaps a good approach would be to add an additional subsidiary error code to the "extra" dictionary.