When referencing a specific path in a WC state descriptor, throw a more meaningful exception if that path doesn't exist in the descriptor.
When referencing a specific path in a WC state descriptor, throw a
more meaningful exception if that path doesn't exist in the
descriptor.

* subversion/tests/cmdline/svntest/wc.py
  (State.tweak): Add some text to the error raised when a path in ARGS
   doesn't exist.

jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion