mongodb_user: properly guard user adding with try...except (#2582)

The user adding part is not properly guarded by a try...except
block, so pymongo exceptions can escape from it.
Also there's a double-guarding where roles are given.

Fixes: #2575

lib/ansible/modules/extras/database/misc/mongodb_user.py

@@ -192,11 +192,7 @@ def user_add(module, client, db_name, user, password, roles):
     if roles is None:
         db.add_user(user, password, False)
     else:
-        try:
         db.add_user(user, password, None, roles=roles)
-        except OperationFailure, e:
-            err_msg = str(e)
-            module.fail_json(msg=err_msg)
 
 def user_remove(module, client, db_name, user):
     exists = user_find(client, user, db_name)
@@ -339,6 +335,7 @@ def main():
         if password is None and update_password == 'always':
             module.fail_json(msg='password parameter required when adding a user unless update_password is set to on_create')
 
+        try:
             uinfo = user_find(client, user, db_name)
             if update_password != 'always' and uinfo:
                 password = None
@@ -348,7 +345,6 @@ def main():
             if module.check_mode:
                 module.exit_json(changed=True, user=user)
 
-        try:
             user_add(module, client, db_name, user, password, roles)
         except OperationFailure, e:
             module.fail_json(msg='Unable to add or update user: %s' % str(e))