membership unit test checks joinability and player limit; post.create() checks allow_post

amanuensis/backend/membership.py

@@ -2,6 +2,7 @@
 Membership query interface
 """
 
+from _pytest.python_api import raises
 from sqlalchemy import select, func
 
 from amanuensis.db import DbContext, Membership
@@ -41,7 +42,8 @@ def create(
     lex: Lexicon = db(
         select(Lexicon).where(Lexicon.id == lexicon_id)
     ).scalar_one_or_none()
-    assert lex, "could not find lexicon"
+    if not lex:
+        raise ArgumentError("could not find lexicon")
 
     # Verify lexicon is joinable
     if not lex.joinable: