Unhelpful error message when using rewrites as the base (tupleset) of a from

[rhamzeh]

model
  schema 1.1
type user
type org
  relations
    define owner: [user]
    define member: [user]
    define can_add_member: owner
type team
  relations
    define parent: [org]
    define member: [user]
    define can_add_member: can_add_member from parent
type group
  relations
    define parent: [team]
    define org: parent from parent
    define member: [user]
    define can_add_member: can_add_member from org

The language server shows an unhelpful ModelValidationError:

`can_add_member` is an impossible relation for `group` (no entrypoint).

The actual error should inform the user the because group -> org has a rewrite (in this case tuple to userset, but computed usersets or a type restriction with a userset should also fail), it cannot be used as the source of a from (cannot be used as a tupleset).

The actual error message should be discussed.

The API shows a much more helpful message in similar situations:

the 'group#org' relation is referenced in at least one tupleset and thus must be a direct relation