Fix issue #4118 (wrong error message if wc.db is missing, or SQL error).

* subversion/libsvn_wc/wc_db_wcroot.c (svn_wc__db_wcroot_parse_local_abspath):
   Handle several cases where we could not find a valid .svn/wc.db, but
   .svn/entries is also obviously not valid for a pre-wc-ng working copy.

* subversion/tests/cmdline/wc_tests.py
  (status_with_various_degrees_of_broken_wc): New test case for the
   failure modes detected by this change.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1428618 13f79535-47bb-0310-9956-ffa450edef68