Make maintainer-mode stack traces less cluttered.

Suggested by: cmpilato

* subversion/libsvn_subr/error.c
  (print_error): Print a placeholder sometimes, and note in a comment that
    assumptions we thereby make about our caller.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1467440 13f79535-47bb-0310-9956-ffa450edef68