Implement which-error.py in C.

* subversion/include/svn_error.h
  (svn_error_symbolic_name): Declare new API.

* subversion/include/svn_error_codes.h
  (SVN_ERROR_START, SVN_ERRDEF, SVN_ERROR_END):
    Include the error name as a string, in SVN_ERROR_BUILD_ARRAY mode only.

* subversion/libsvn_subr/error.c
  (struct err_defn): Add a member, also add comments.
  (svn_error_symbolic_name): Implement new API.

* subversion/tests/libsvn_subr/error-test.c
  (test_error_symbolic_name): New test.
  (test_funcs): Run it, passing.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1464570 13f79535-47bb-0310-9956-ffa450edef68