Are there any free standalone crawlers that I can use apart from Google webmaster to analyze errors?
Something that shows me all 404's, errors etc
Webmasters Stack Exchange is a question and answer site for pro webmasters. It only takes a minute to sign up.Sign up to join this community
Xenu's Link Sleuth is what you're probably looking for
Another crawling tool which is free for upto 500 urls is Screaming Frog SEO Spider.
If you're on a Mac, try Integrity, which is free and works great.