[Ur] Indicating failure in uw_register_transactional callbacks

[Chris Double]

In the commit callback that's passed to uw_register_transactional how
does one indicate failure of the transaction? I can't call uw_error
since that requires a context and I don't have one. The documentation
for uw_register_transactional says not to use any context data so even
if I pass it in I can't use it (is that right?). I'm assuming that
calling any uw_... functions inside of the commit callback is a bad
idea?

Chris.
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