lone custom outputs ignored

[hjoliver]

This validates OK, and :x is ignored in the completion expression (same for a:x?):

[scheduling]
   [[graph]]
        R1 = "a:x"
[runtime]
   [[a]]  # no outputs defined

But triggering another task off the output results in a warning:

[scheduling]
   [[graph]]
        R1 = "a:x => b"
[runtime]
   [[a]]  # no outputs defined
   [[b]]

WorkflowConfigError: Undefined custom output: a:x

[wxtim]

@hjoliver Do you want to make validation fail?

[scheduling]
   [[graph]]
        R1 = "a:x"
[runtime]
   [[a]]  # no outputs defined

fail validation?

[wxtim]

I think that I've convinced myself that this graph should raise an error - but not that error. And, that foo:x or bar => foo:x should raise an error whether or not outputs are defined. This is becuase a qualifier at the right hand end of a graph sequence will never have any effect, and is therefore always a user error. If the user expects it to have any effect they are wrong, and Cylc should fail harder.