handle alembic exception
Alembic might throw an CommandError exception if an older CodeChecker tries to connect to a database with a newer db schema in it.
Handle alembic errors in the [database_handler.py](https://github.com/Ericsson/codechecker/blob/03fd2a04fe8bfad4c56d586130efcbcc6955a440/codechecker_lib/database_handler.py#L64) while trying to create or migrate the database schema.

diff --git a/codechecker_lib/database_handler.py b/codechecker_lib/database_handler.py
index 3eb4f111..a842a737 100644
--- a/codechecker_lib/database_handler.py
+++ b/codechecker_lib/database_handler.py
@@ -15,6 +15,7 @@ import time
 from abc import ABCMeta, abstractmethod
 
 import sqlalchemy
+from alembic.util import CommandError
 from alembic import command, config
 from sqlalchemy import event
 from sqlalchemy.engine import Engine
@@ -96,6 +97,11 @@ class SQLServer(object):
         except sqlalchemy.exc.SQLAlchemyError as alch_err:
             LOG.error(str(alch_err))
             sys.exit(1)
+        except CommandError as cerr:
+            LOG.error("Database schema and CodeChecker is incompatible."
+                      "Please update CodeChecker.")
+            LOG.debug(str(cerr))
+            sys.exit(1)
 
     @abstractmethod
     def start(self, db_version_info, wait_for_start=True, init=False):
['codechecker_lib/database_handler.py']