Refactor SQLPatternMatcher::query_info_to_pattern to return Result instead of an error-carrying SQLQuery

[milindsrivastava1997]

sqlpattern_matcher.rs: SQLQuery carries its own error/msg fields and is_valid(), and query_info_to_pattern returns a plain SQLQuery rather than Result.
Change it to return Result<SQLQuery, (QueryError, String)>, matching flatten_query_info's own convention, and drop error/msg/is_valid()/invalidate_query() from SQLQuery.