Follow-up to r1467261: don't ignore svn_error_t* returns in svn-bench.

* tools/client-side/svn-bench/svn-bench.c
  (sub_main): handle return value of svn_cl__help()

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1467262 13f79535-47bb-0310-9956-ffa450edef68