hook script error messages

Is there a way to set the error message that comes back to the client
when a (pre-commit) hook script fails?

I'm testing with TortoiseSVN, and when the template pre-commit script
fails, the client gets a message like:

   Commit failed (details follow):
   MERGE request failed on '/some/repos/path/'

   'pre-commit' hook failed with error output:

Looks like there's no output from the script... how could I generate
something useful for the user?

-- 
Neil Gower
Brainbox Games
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