Support symbolic names for APR error codes, too.

* build/generator/gen_base.py
  (GeneratorBase.write_errno_codes): Parse aprerr.txt into errorcode.inc.

* subversion/libsvn_subr/error.c
  (svn_error_symbolic_name): Use svn__apr_errno (from errorcode.inc).

* tools/dev/which-error.py
  (get_errors): Add a cross-reference comment.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1478606 13f79535-47bb-0310-9956-ffa450edef68