Add a missing explanation into an error message.
Add a missing explanation into an error message.

* subversion/libsvn_client/prop_commands.c
  (svn_client_revprop_set2): When refusing to set the author property because
    it contains a newline, make the error message say what the problem was.


git-svn-id: f8a4e5e023278da1e04e203c7fe051e3c4285d88@879825 13f79535-47bb-0310-9956-ffa450edef68

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