In maintainer mode, print symbolic error names in stack traces:

  % $svn ci
  subversion/svn/svn.c:2857: (apr_err=SVN_ERR_WC_UPGRADE_REQUIRED)
  svn: E155036: Please see the 'svn upgrade' command
  subversion/libsvn_wc/wc_db_wcroot.c:743: (apr_err=SVN_ERR_WC_UPGRADE_REQUIRED)
  subversion/libsvn_wc/wc_db_wcroot.c:317: (apr_err=SVN_ERR_WC_UPGRADE_REQUIRED)
  svn: E155036: The working copy at '/tmp/wc-1.6' is too old (format 10) to \
  work with client version '1.8.0-dev (under development)' (expects format 31). You need to upgrade the working copy first.

* subversion/libsvn_subr/error.c
  (print_error): Print the symbolic name, using svn_error_symbolic_name().

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1465157 13f79535-47bb-0310-9956-ffa450edef68