Add a new "tracing" mechanism for svn devs. When SVN_DEBUG is defined (which, in turn, defines SVN_ERR_TRACING), every time an SVN_ERR() sees an error, then a new error is chained into the sequence automatically showing the current file/line which is causing the problem.
Add a new "tracing" mechanism for svn devs. When SVN_DEBUG is defined
(which, in turn, defines SVN_ERR_TRACING), every time an SVN_ERR() sees an
error, then a new error is chained into the sequence automatically showing
the current file/line which is causing the problem.

* subversion/include/svn_error.h:
  (SVN_ERR_TRACING): defined when SVN_DEBUG is defined (ie. svn devs)
  (SVN_ERR): provide one of two forms, to enable tracing


git-svn-id: f8a4e5e023278da1e04e203c7fe051e3c4285d88@877351 13f79535-47bb-0310-9956-ffa450edef68

wbond/subversion,wbond/subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,wbond/subversion,wbond/subversion,YueLinHo/Subversion,YueLinHo/Subversion,wbond/subversion,YueLinHo/Subversion,YueLinHo/Subversion,wbond/subversion,wbond/subversion,YueLinHo/Subversion