Change generate-assertions http response to status only

[vteague]

See meeting notes - assume the dashboard gets info from the database and instead just give it a 4-valued ENUM to tell it whether assertion generation succeeded or failed, and whether it's worth redoing.

[vteague]

Actually this should apply to #raire-service too - might as well return status only from the raire-service endpoint.

Change GenerateAssertionsResponse to have two booleans: one for whether assertion generation succeeded, and one for whether it is worth retrying.