Add 'const' to the error input parameter of some error-related APIs that
don't clear or modify the error, for clarity.

A follow-up to r1584610 which did this to a function in svnserve, which gave
warnings indicating we need to do the same to some lower level functions.

* subversion/include/private/svn_error_private.h,
  subversion/include/svn_error.h,
  subversion/libsvn_subr/error.c
  (svn_err_best_message,
   svn_handle_warning2,
   svn_error__is_tracing_link): Make the input error pointer 'const'.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1584613 13f79535-47bb-0310-9956-ffa450edef68