postinstall: raise a proper error when trying to use e.g. 'admin' as the first username which will conflict with the admins group mail aliases

src/tools.py

@@ -161,7 +161,7 @@ def tools_postinstall(
         assert_password_is_compatible,
     )
     from yunohost.domain import domain_main_domain
-    from yunohost.user import user_create
+    from yunohost.user import user_create, ADMIN_ALIASES
     import psutil
 
     # Do some checks at first
@@ -174,6 +174,9 @@ def tools_postinstall(
             raw_msg=True,
         )
 
+    if username in ADMIN_ALIASES:
+        raise YunohostValidationError(f"Unfortunately, {username} cannot be used as a username", raw_msg=True)
+
     # Check there's at least 10 GB on the rootfs...
     disk_partitions = sorted(
         psutil.disk_partitions(all=True), key=lambda k: k.mountpoint