The 5xx (Server Error) class of status code indicates that the server
is aware that it has erred or is incapable of performing the
requested method. Except when responding to a HEAD request, the
server SHOULD send a representation containing an explanation of the
error situation, and whether it is a temporary or permanent condition.
The 503 (Service Unavailable) status code indicates that the server
is currently unable to handle the request due to a temporary overload
or scheduled maintenance, which will likely be alleviated after some
delay. The server MAY send a Retry-After header field... to suggest an appropriate amount of time for the
client to wait before retrying the request.
a client MUST
understand the class of any status code, as indicated by the first
digit, and treat an unrecognized status code as being equivalent to
the x00 status code of that class