Raise better error when through relationship is not defined

```crystal
class Group < BaseModel
table :groups do
column title : String
# missing has_many memberships : Membership
has_many users : User, through: :memberships
end
end

class User < BaseModel
table :users do
column email : String
# missing has_many memberships : Membership
has_many groups : Group, through: :memberships
end
end

class Memebership < BaseModel
table :memeberships do
belongs_to group : Group
belongs_to user : User
end
end
```

When trying to join memberships, there's a compilation error:

```crystal
groups = GroupQuery.new.join_users

# undefined local variable or method 'inner_join_memberships' for GroupQuery
```

If you're not familiar with joins in Avram, it's not obvious what you're missing as everything seems to be in place. We should detect this and show a better error message that may help lead the user to the solution.

Start by reproducing the GroupQuery.new.join_users example and inspect how Avram generates the through-relationship join methods. Trace the missing inner_join_memberships error and add detection that reports the absent relationship clearly; done when the example produces an actionable error instead of the undefined method compilation error.

Written by the indexing model from the issue text.