Note: If you're actually seeing HTTP error codes greater than 415 and less than 500, they're bogus. Most likely your web access is going through Winproxy, which returns "HTTP" error codes such as 425... which actually appears to be borrowed from the FTP error codes, and is not legitimate in HTTP.
If you want standard codes, go complain to the perpetrators of Winproxy, OSitis (literally, an inflammation of the operating system).
Hey, looky: OSitis documented their WinPoxy error codes. Finally.
We're all familiar with 404, and perhaps 403... but have you ever wondered what the other HTTP error codes are?
Here, for your edification, is the list of RFC2068 (HTTP 1.1) client error (400-series) codes:
In addition, several extended client error codes exist, though they are not widely used:
NOTE: these extended error codes are bogus (just in case you hadn't figured that out already). RFC2068 does not in fact define 400-series error codes above 415. If you're getting an error 425, it's either (a) an FTP error, not HTTP, or (b) a bogus error code returned by a proxy which doesn't conform to the HTTP/1.1 standard.
For the authoritative reference, see http://www.ietf.org/rfc/rfc2068.txt.