The construction of error objects is relatively expensive (due to the
NLS libs involved).  So, if files tend to not be present, check whether
they actually exist before opening them.

* subversion/libsvn_fs_fs/lock.c
  (read_digest_file): most times, there are no locks

* subversion/libsvn_repos/hooks.c
  (svn_repos__parse_hooks_env): most people don't configure a hook env

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1544721 13f79535-47bb-0310-9956-ffa450edef68