Return an error if the dump target URL is not present in HEAD.

* subversion/svnrdump/svnrdump.c
  (main): Check that the dump target actually exists. If not, error out
    immediately.

* subversion/tests/cmdline/svnrdump_tests.py
  (dump_non_existent_url): New test.
  (test_list): Add reference to new test.

Patch by: Vijayaguru G <vijay{_AT_}collab.net