site: Fix bugs when resolver request submission fails

Previously, if adns_submit failed:
 - the struct query in resolver.c was leaked
 - nothing was logged
 - the return value from resolver->request was ignored so the site
   state machine would hang

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>
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Changes in v2:
  * Fixed typo in commit message.