Conditional newline in prompt

[Christoph Wurm <christoph@xxxxxxxx>]

Hello,

I would like to display the exit code of a command if it is not 0 on a 
line of its own, followed by the prompt on the next line.

Is this possible?

I was able to come up with the following:

PROMPT="%0(?..%?)
%{$fg[green]%}%n@%m%{$reset_color%}:%~ %{$fg[red]%}%#%{$reset_color%} "

However, this prints a newline after every command. Is there some 
equivalent of '\n' that can be used inside the conditional substring? Or 
some other way to do this?

Kind regards
Christoph