Arvados

When I tried to use the same collection name twice, the error message (the uniqueness constraint violation exception from our PostgreSQL driver, rendered by X-Editable) looked like an internal error message that wasn't intended to be shown to an end user.

1. API server should provide an error message phrased in Arvados terms, suitable for displaying to users, like errors.add :name, "must be unique within this project"
2. Workbench should render errors like this without strange formatting -- e.g., "Error: name must be unique" instead of <ArvadosApiClient> 422 {"name", "must be unique"}