add isErrorPage method in parser

[mike820324]

as title suggest, each parser should have a isErrorPage method instead of using try/catch in crawl method.

[mike820324]

current error handling still not enough. when crawling mh.99770.cc, it will still stop with undefined exception type'

[mike820324]

it turns out that I only add parser error handling in dmeden parser. The newest commit should add proper error handling in other parsers.