Search the chain of sp_rcontexts recursively for handlers. If one is found,
  it will be detected in the sp_head::execute() method at the corresponding
  level.


mysql-test/r/sp.result:
  New test case for BUG#7049.
  Note that the spurious warnings in the BUG#12379 test now are gone (as expected).
mysql-test/t/sp.test:
  New test case for BUG#7049.
sql/sp_head.cc:
  Link sp_rcontexts to allow catching errors across invokation boundaries.
  (Also fixed print method for the hreturn instruction.)
sql/sp_rcontext.cc:
  Link sp_rcontexts to allow catching errors across invokation boundaries.
  If a handler is not found in the current sp_rcontext, recurse into the previous ones (if any).
sql/sp_rcontext.h:
  Link sp_rcontexts to allow catching errors across invokation boundaries.