taskd.pl: do not abandon unloadable clones
If a clone failes to start because the project cannot be loaded,
check to see if it really does exist and has been marked for cloning.
If it has, then helpfully touch .clone_failed so that it's possible
to attempt to restart the clone later.
This shouldn't normally be necessary, but if something on the system
goes temporarily wonky that causes the project load to fail the clone
would otherwise be left hanging -- marked for cloning, but not actually
cloning and not restartable because it's not yet failed.
Signed-off-by: Kyle J. McKay <mackyle@gmail.com>