Heres my best attempt at all this:
400 Bad File Request Usually means the syntax used in the URL is incorrect (e.g., uppercase letter should be lowercase letter; wrong punctuation marks).
401 Unauthorized Server is looking for some encryption key from the client and is not getting it. Also, wrong password may have been entered. Try it again, paying close attention to case sensitivity.
403 Forbidden/Access Denied Similar to 401; special permission needed to access the site -- a password and/or username if it is a registration issue. Other times you may not have the proper permissions set up on the server or the site's administrator just doesn't want you to be able to access the site.
404 File Not Found Server cannot find the file you requested. File has either been moved or deleted, or you entered the wrong URL or document name. Look at the URL. If a word looks misspelled, then correct it and try it again. If that doesn't work backtrack by deleting information between each backslash, until you come to a page on that site that isn't a 404. From there you may be able to find the page you're looking for.
408 Request Timeout Client stopped the request before the server finished retrieving it. A user will either hit the stop button, close the browser, or click on a link before the page loads. Usually occurs when servers are slow or file sizes are large.
500 Internal Error Couldn't retrieve the HTML document because of server-configuration problems. Contact site administrator.
501 Not Implemented Web server doesn't support a requested feature.
502 Service Temporarily Overloaded Server congestion; too many connections; high traffic. Keep trying until the page loads.
503 Service Unavailable Server busy, site may have moved ,or you lost your dial-up Internet connection.
Connection Refused by Host Either you do not have permission to access the site or your password is incorrect.
File Contains No Data Page is there but is not showing anything. Error occurs in the document. Attributed to bad table formatting, or stripped header information.
Bad File Request Browser may not support the form or other coding you're trying to access.
Failed DNS Lookup The Domain Name Server can't translate your domain request into a valid Internet address. Server may be busy or down, or incorrect URL was entered.
Host Unavailable Host server down. Hit reload or go to the site later.
Unable to Locate Host Host server is down, Internet connection is lost, or URL typed incorrectly.
Network Connection Refused by the Server The Web server is busy.
