The 4xx class of status code is intended for cases in which the
client seems to have erred. Except when responding to a HEAD request,
the server SHOULD include an entity containing an explanation of the
error situation, and whether it is a temporary or permanent
condition. These status codes are applicable to any request method.
User agents SHOULD display any included entity to the user.
Note: If the client is sending data, a server implementation using
TCP should be careful to ensure that the client acknowledges
receipt of the packet(s) containing the response, before the server
closes the input connection. If the client continues sending data
to the server after the close, the server's TCP stack will send a
reset packet to the client, which may erase the client's
unacknowledged input buffers before they can be read and
interpreted by the HTTP application.