Correct an error message and use a #define instead of a magic number.
No functional change.

* subversion/libsvn_ra_svn/marshal.c
  (ITEM_NESTING_LIMIT): define new constant
  (read_item): the problem is recursion not data size; use new constant

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1486014 13f79535-47bb-0310-9956-ffa450edef68