Provide an error message to users if all their update targets are not working
copies rather than just telling them we skipped them.

The existing behavior is terribly confusing.  Why did we skip it?  The behavior
is there to allow things like `svn up *` where some of the paths end up not
being working copies.  But it comes at a tremendous cost.  This change tries
to walk the line by providing an error if all of the targets fail due to not
being working copies.  There still will some possible confusion if only some
of the targets fail due to not being working copies since you'll still get
only the skipped behavior, but without changing the notification API I don't
see a way around it.

* subversion/libsvn_client/update.c
  (svn_client_update4): return an error if all the targets aren't wcs.  It
    will still generate the skip message so users can still see what their
    targets are.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1565388 13f79535-47bb-0310-9956-ffa450edef68