Commented by: @AlexPeshkoff

There were some known (and already fixed) bugs in user management in Alpha1. Currently all real job (like in many other DDL) is performed at commit time, and the fact that errors from DDL from EXECUTE STATEMENT are ignored is not user management specific. This is what we will get with beta1 (I use manual commit to make it better visible, DatabaseAccess is restricted in firebird.conf):

#⁠ ./isql employee
Database: employee
SQL> set term ^;
SQL> EXECUTE BLOCK AS BEGIN
CON> EXECUTE STATEMENT 'create shadow 1 ''/tmp/qq''';
CON> WHEN ANY DO BEGIN END END^
-- no error
SQL> commit^
Statement failed, SQLSTATE = 28000
Access to additional database file "/tmp/qq" is denied by server administrator
-- error when doing commit
SQL> rollback^

But there is a way to handle errors - using new Autonomous Transactions:

SQL> EXECUTE BLOCK AS BEGIN
CON> in autonomous transaction do
CON> EXECUTE STATEMENT 'CREATE USER TEST PASSWORD ''test''';
CON> WHEN ANY DO BEGIN END END^
SQL> commit^
-- no errors displayed

Without error handler error is certainly visible:

SQL> EXECUTE BLOCK AS BEGIN
CON> in autonomous transaction do
CON> EXECUTE STATEMENT 'CREATE USER TEST PASSWORD ''test''';
CON> END^
Statement failed, SQLSTATE = HY000
add record error
SQL> commit^
-- pay attention - error displayed not in commit, but in EXECUTE BLOCK

Be careful - using autonomous transaction you will not be able to rollback user creation (like any other statement).