Do not ignore error returned from 'partial getter' in membuffer cache when 
compiled to use simple mutex instead of read/write lock.

* subversion/libsvn_subr/cache-membuffer.c
  (unlock_cache): Pass ERR argument to svn_mutex__unlock() call.

* subversion/tests/libsvn_subr/cache-test.c
  (test_membuffer_serializer_error_handling): New test to reproduce the 
   issue.
  (raise_error_deserialize_func, raise_error_partial_getter_func): Helpers 
   for test_membuffer_serializer_error_handling test.
  (test_funcs): Add test_membuffer_serializer_error_handling() test to 
   the list.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1647339 13f79535-47bb-0310-9956-ffa450edef68