I'm surprised this isn't customizable via group policy, or some other
administrative tool - whenever a web site goes down (particularly if it
happens to be our uers' home page) our help desk is inevitably barraged with
"my computer is down" requests.
At any rate, I tried using ResHacker to modify this DLL. Then I replaced
the one in \windows\system32\dllcache, and in \windows\system32 - however IE
seems to still have managed the out-of-the-box message, even though the URL
of the error still references and looking at \windows\system32\shdoclc.dll
with ResHacker shows my modifications in 23\DNSERROR.HTM\1033.