fix: issue2551374. Error handling for filter expressions. Errors in filter expressions are now reported. The UI needs some work but even the current code is helpful when debugging filter expressions. mlink_expr: defines/raises ExpressionError(error string template, context=dict()) raises ExpressionError when it detects errors when popping arguments off stack raises ExpressionError when more than one element left on the stack before returning also ruff fix to group boolean expression with parens back_anydbm.py, rdbms_common.py: catches ExpressionError, augments context with class and attribute being searched. raises the exception for both link and multilink relations client.py catches ExpressionError returning a basic error page. The page is a dead end. There are no links or anything for the user to move forward. The user has to go back, possibly refresh the page (because the submit button may be disalbled) re-enter the query and try again. This needs to be improved. test_liveserver.py test the error page generated by client.py db_test_base unit tests for filter with too few arguments, too many arguments, check all repr and str formats.