make server respond consistent error messages

[mren]

as of 089f9c998d895d19ab4ef4091a81a4267386f656 the server does only ends the request when an error is detected. which error messages should we return? is there a spec for the xapi?

[booo]

The end event should close the connection.

We should return 500 on error if nothing was written to the socket. Else we should just close the connection.

[slomo]

I think the http standard is clear about this. http://en.wikipedia.org/wiki/HTTP_status_code

400 => User errors 500 => Unrecoverable system errors

[mren]

with yesterdays commit (4e8b6b012bbf1f53e9200db3fe4ce218accc7d1c) we have 400 for a parse error and 500 for a database error