Send an error message to the client.
If any error messages are sent before the references are advertised to the client, the errors will be sent instead of the advertisement and the receive operation will be aborted. All clients should receive and display such early stage errors.
If the reference advertisements have already been sent, messages are sent in a side channel. If the client doesn't support receiving messages, the message will be discarded, with no other indication to the caller or to the client.
{@link PreReceiveHook}s should always try to use {@link ReceiveCommand#setResult(Result,String)} with a result status of{@link Result#REJECTED_OTHER_REASON} to indicate any reasons forrejecting an update. Messages attached to a command are much more likely to be returned to the client.
@param what string describing the problem identified by the hook. The string must not end with an LF, and must not contain an LF.