Fix: Use OperationalError from flask_sqlalchemy. (Redmine issue: #6205)

lib/mentat/services/sqlstorage.py

@@ -29,7 +29,6 @@ __credits__ = "Pavel Kácha <pavel.kacha@cesnet.cz>, Andrea Kropáčová <andrea
 import copy
 from flask import Flask
 import flask_sqlalchemy
-from sqlalchemy.exc import OperationalError
 
 #
 # Custom libraries
@@ -50,7 +49,7 @@ class RetryingQuery(flask_sqlalchemy.orm.Query):
         for _ in range(2):
             try:
                 return super()._execute_and_instances(querycontext)
-            except OperationalError:
+            except flask_sqlalchemy.orm.exc.sa_exc.OperationalError:
                 self.session.close()
                 continue