Don't swallow transaction errors (#236)

A change proposed by @kjohnsen from #216.

You can read more starting with [this comment](#216 (comment)).

flask_migrate/templates/flask/env.py

@@ -74,10 +74,13 @@ def process_revision_directives(context, revision, directives):
                       target_metadata=target_metadata,
                       process_revision_directives=process_revision_directives,
                       **current_app.extensions['migrate'].configure_args)
-
+    
     try:
         with context.begin_transaction():
             context.run_migrations()
+    except Exception as exception:
+        logger.error(exception)
+        raise exception
     finally:
         connection.close()