Change an assert() which frequently triggers on corrupt repositories into a useful error.
Change an assert() which frequently triggers on corrupt repositories into
a useful error.

* subversion/libsvn_fs_base/reps-strings.c
  (rep_undeltify_range): Change assert() to error.
  (rep_read_range): Wrap error in useful information.


git-svn-id: f8a4e5e023278da1e04e203c7fe051e3c4285d88@855880 13f79535-47bb-0310-9956-ffa450edef68

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