Hitting the Pickaxe book, it says that returns from a block will cause
this error. (Why? I don't know.
Nor should you care.
So ... how does one exit gracefully from a block prior to falling off
the end of the block?
One doesn't AFAIK. In the general case, you could use a conditional
statement or raise an exception. In your case, I'd advise using a
method callback instead of a block.
Best,