Return an error if trying to set a summary with newlines.
    
At the moment, it's possible to create a review request that has a summary with
a newline in it. This works okay until RB tries to send email with that summary
as a subject, at which point it will blow up with an ugly backtrace.
    
I considered normalizing, but that's uglier. Since this is something that can
only be done via the JSON API, we might as well have the API return an error
instead.