Fix theoretical error leak returned from cache partial setter callback.

* subversion/libsvn_subr/cache-membuffer.c
  (membuffer_cache_set_partial_internal): Do not ignore error from partial 
   setter callback.
* subversion/tests/libsvn_subr/cache-test.c
  (test_membuffer_serializer_error_handling): Extend test to cover another 
   case.
  (raise_error_partial_setter_func): New helper for 
   test_membuffer_serializer_error_handling()


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1647372 13f79535-47bb-0310-9956-ffa450edef68