Fix issue #2505: make switch continue after deleting locally modified
directories, as it update and merge do.

* subversion/libsvn_wc/update_editor:
  (leftmod_error_chain): tolerate SVN_ERR_WC_LEFT_LOCAL_MOD. The
   whole error is cleared and SVN_NO_ERROR is returned if
   SVN_ERR_WC_LEFT_LOCAL_MOD error is found in the error chain. And
   we don't delete logfile in that case.
  (do_entry_deletion): svn_wc_remove_from_revision_control() is called
   with instant_error = FALSE.

* subversion/tests/cmdline/switch_tests.py:
  (tolerate_local_mods): New test.
  (test_list): Run the new test.